Note: due to links not being allowed, the urls at the bottom have intentionally been broken up, just remove the spaces.
Preface:
First of all this is merely a guide to help you. You’re not under any obligation to RP your Blackrock orc this way, nor do you risk being seen as a bad roleplayer, it’s just a helpful tools. And it’s not the best guide either 
I have focused mainly on MU Blackrock orcs, so the lack of including the AU Blackrock clan was unfortunate. Definitely see Gardahl’s post below.
A brief introduction.
So what traits defines the Blackrock clan?
Well they’re a clan consisting of craftsman and warriors mainly, and they’ve shown patience, pragmatism and tactical thinking throughout their history.
A Blackrock orc, while honourable like many other orcs, isn’t governed by it completely and is able to see the larger picture at times. Their culture also reflects that they’re quick to adapt to changes and they can be seen as a more progressive clan, willing to evolve. If any clan is willing to make friends with former enemies (if only for a short moment) it would be the Blackrock clan.
I will be going through the following points (and some of my own speculation) that demonstrates the traits of the Blackrock clan:
Patience
Pragmatism
Tactical aspect
Patience
In the book “Lord of the Clans” Thrall meets Doomhammer, they duel, it’s super cool and all that. However Thrall is just one Orc, who had arrived recently to the Frostwolf camp.
So why didn’t Doomhammer, who had planned the raids against the interment camps, attack before Thrall arrived? Why did he wait? A: he knows more than any other Orc, that humans can’t be underestimated. They are weaker physically yes, but they got the numbers and a fighting spirit to rival any orc.
So rushing in as soon as a plan is ready would have been foolish and likely have ended up in a catastrophe; Instead Orgrim waited for a good opportunity to attack. Now I know I’m speculating, but is it really that unreasonable to assume that Doomhammer could show patience and perhaps wait for the enemy to become lazy and complacent?
“Because I needed to be by myself, to analyse what had happened. To think. To remind myself who I was, who we were as a people. From time to time, I would do as I have done this night. I would venture forth to the campfires, accept their hospitality, listen to their experiences, and learn.” He paused. “I know the insides of human prisons, as you do. I was captured and kept as an oddity by King Terenas of Lordaeron for a time. I escaped from his palace, as you escaped from Durnholde. I was even in an encampment. I know what it is like to be that broken, that despairing. I almost became one of them.” (Golden 2001)
By the time he met Thrall the guards had become complacent and lazy indeed, which would have certainly increased the odds of a successful raid. Then his patience also got rewarded, by discovering Durotan’s presumed dead son; who was not only alive, but had rekindled (largely thanks to Drek’thar) the orcs relationship with the elements. A strong weapon indeed.
B: Blackrock orcs are famed blacksmiths amongst the orcs and the Horde. You don’t get to become a good blacksmith by half-arsing your work, and producing rushed, and shoddy products. Blacksmithing like any trade that involves skill, requires patience, precision and experience. Could you imagine an impatient orc, be a good blacksmith?
In the book rise of the Horde they are also quoted as: It helped that both the Frostwolves and the Blackrocks were both traditionally even-tempered orc clans. (Golden 2006)
Followed by another quote: but the hunting skills of the Blackrock were legendary. (Golden 2006)
And one about their leader: He [Blackhand] knew well that most of the orcs, even those of his own clan, thought him hotheaded and impetuous, but he knew the value of patience. (Golden 2006)
Next let us look at pragmatism.
Pragmatism
The best way to summarise why the Blackrock clan had a pragmatic view of the world, is to once again take a look at the most prominent Blackrock orc, Blackhand, Doomhammer and to a lesser degree Rend and Maim, and use them as our template.
Blackhand, for example, hated the idea of allying with the Ogres, but he was wise enough (though it should be noted he was partly driven by this ego) to recognise the importance of having the lumbering brutes as troops to counter the mounted knights.
“Absolutely not!” cried Blackhand. He closed the distance between himself and Gul’dan in two strides, towering over the smaller orc. It took every ounce of bravado Gul’dan had not to retreat from that fearsome face shoved to within an inch of his.
“Come now, Blackhand” Gul’dan soothed. “Calm yourself and listen to what I am saying. You will be the one to benefit most from this, after all.” That got him. Blackhand growled, snorted, and stepped back. Gul’dan did his best not to look obviously relieved. (Golden 2006)
Doomhammer, who didn’t have a lust for power, like Blackhand also saw the value in them, hence why he didn’t get rid of the Ogres after he took over the mantle as Warchief.
He had been disgusted when Blackhand had forged an alliance with the monstrous creatures, though he admitted they had their use in combat. But he still did not like or trust them. (Rosenberg 2007)
When Doomhammer assassinated Blackhand, or ambushed him (it depends on how you like to see it, but he’s known as the “Backstabber” for a reason), he allowed Gul’dan to live.
Now see this is where you’ll see why he’s considered a pragmatic orc. Orgrim knew that the Alliance greatest weapon against them was sorcery, but having the shadow council running about would constitute that Gul’dan would eventually try to seize power again. So naturally robbing Gul’dan of his council was elementary, and killing Gul’dan would’ve been logical too, if Gul’Dan had not offered to create the Death Knights at the last moment. Now before you think it was stupid of Doomhammer to spare him, remember hindsight is 20-20 and we (armed with meta knowledge) unlike Orgrim, knew what was going on in Gul’dan’s mind. No one in the Horde knew, other than those Gul’dan’s closed circle, about the Great Betrayer’s true intentions. So in Doomhammer’s mind, and to anyone else for that matter, Gul’dan is done. He has no where to go, his precious council are all dead and there’s no way he would be able to command the Horde. Rend and Maim would most likely not serve him either, considering he was the one who manipulated their father, and the alliance would execute him on the spot. So his only option is to serve Doomhammer and deliver his promise of death knights to combat the magic of the alliance. You don’t throw away something that you can still use, which is a trait that defines the Blackrock clan. The overall picture of victory trumped personal honour for Orgrim.
Orgrim made it clear to me that the Horde was under his control, and that he was not as easily swayed as his predecessor. The gleam in his eye and the steel at his side bespoke of his intentions, but I would not be defeated so easily. While he may have held the upper hand, I reminded him that with the death of the Warlocks, I was the last true sorcerer within the Horde. Orgrim, made impudent by his victory, agreed that perhaps I could prove useful, and agreed to let me live - by his good graces.(Metzen 1995)
And it turned out well too, the death knights were an efficient counter to the mages of the Alliance, and the Horde was on the verge of winning the second war. No one could possible know that Gul’dan would suddenly, at the most critical point of the war, go rogue and take a significant amount of the Horde with him to raise the tomb of Sargeras. It made no sense, why not do it after the war was over? But Gul’dan, as we know is one selfish and greedy bastard and cared not for the Horde being victorious, but for his own ambitious power craving lust. Imagine how the second war would’ve gone, had Doomhammer not sanctioned the creation of the Death Knights.
But that is for another matter, the point is, although Gul’dan was dangerous and is the one that pretty much cursed the Orcs his usefulness was still so significant that Doomhammer, rather than satisfy the honour of his people and execute him on the spot, kept him alive for strategic purposes to get a weapon against the mages/sorcerers of the Alliance.
We can also use Rend and Maim as an example, rather than outright rebelling against Doomhammer after he killed their father, Blackhand, they accepted what happened, since in-fighting would hurt the Horde’s advances against the alliance. One could speculate that they were planning to kill Orgrim once the Horde had won the war, but regardless they chose to go with Doomhammer as their new warchief and in turn, Doomhammer allowed them to keep their clan.
This brief clan introduction also allows those that want to RP a Black Tooth Grin orc, to learn about the clans history. And while the clan’s differed in minor ways, they still, like the Blackrock clan in general, valued the over all picture over personal honour.
The Black Tooth Grin clan was originally part of the Blackrock clan, but splintered off before the Horde’s passing into Azeroth. After their father was deposed from his position as War Chief by Orgrim Doomhammer, Rend and Maim decided to gain control over their own faction within the Horde without directly opposing the Doomhammer’s ascension to power. The Black Tooth Grin, noted for its custom of each member knocking out one of his own teeth to symbolize his loyalty to the clan, is charged with the protection and preservation of the dreaded Portal. The unity of The Black Tooth Grin may be sorely tested should the opportunity arise for Rend and Maim to reclaim their father’s station and avenge his death. (Metzen 1995)
He and his brother Maim hated Doomhammer for killing their father and taking his place as Warchief. They were among the few who dared to question Doomhammer’s orders. Doomhammer allowed it, both because he knew any explanations he gave would filter back to the rest of the Horde and because the Black Tooth Grin was a large, powerful clan and therefore useful. Besides, the brothers might question actions or decisions but they never disobeyed a direct order, even when they disagreed with it. Doomhammer appreciated that, and was willing to tolerate their questions, up to a point. (Rosenberg 2007)
Tactical Aspect
Now for the tactical aspect. Since the Horde, largely lead by the Blackrock clan, defeated the Draenei, conquered Stormwind and nearly defeated the Alliance during the second war, we can pretty assume that, as the lore suggest they’re quite competent military strategos.
They also understood the importance of resources, supply lines (Warcraft 2 missions) and the value of allies, as we saw in the rescue of Zul’Jin and then successfully convincing him and his forest trolls to join the Horde.
Blackhand knew that many thought him a fool; he preferred to let them think he was nothing more than a strong arm and a powerful sword. It gave him distinct advantages. (Golden 2006)
But Orgrim knew that whatever else his leader [Blackhand] was—and he was many things that Orgrim did not like—he was a good strategist. (Golden 2006)
The rescue of Zuljin.
Our spies report that a band of Human soldiers have captured a war party led by Troll commander Zuljin and have taken them to a secret prison near the township of Hillsbrad.
Seeing an opportunity to place these captives in debt to the Horde, Doomhammer sends you to ransom Zuljin and his Trolls and return them to their own encampment nearby.
The War Chief believes that this raid upon the unsuspecting prison will strike terror into the hearts of those who would dare resist the Horde. (Warcraft 2; mission briefing 1995)
Here we can see, that despite Trolls being completely alien to the Orcs, they still immediately jumped on the oppertunity, to make a friend against their enemies, counting on the age old saying “My enemy’s enemy, is my friend.” Which turned out to be prudent, as they not only got excellent fighters with regenerative abilities, but also good knowledge of the land to use to their advantage in future skirmishes against the Alliance.
Defending Southshore
In preparation for a final strike on Hillsbrad, the War Chief directs you to begin construction of facilities for the Orcish Armada near the Southshore region.
Zuljin and his Trolls, eager to take revenge upon the Humans who imprisoned them, have agreed to aid the Horde by supplying Axethrowers and Destroyers to assist in defending your Southshore operation.
A Shipyard must be constructed in order to build our wave riders and you will need much of the black liquid known as Oil to build your fleet.
Our assault on Hillsbrad cannot begin until your task is completed.(Warcraft 2; mission briefing 1995)
Here we see, probably the best example that demonstrates the strategic thinking of the Blackrock Clan. Instead of rushing to take Hillsbrad, Doomhammer ordered the Horde to secure the oil, to construct the fleet he needed to begin the assault. Not to mention, the quick recognition of the importance of oil.
The conquest of Dalaran.
The hour of judgement is close at hand as the Orcish Hordes stand ready to sweep across this domain like a pestilence and seize the capital of Lordaeron. Standing vigilant above the plains like the ascending arm of twilight itself, is the Violet Citadel of Dalaran. The Citadel - serving as sanctum and haven to the Mages of Lordaeron - is the last barrier between the Orcs and their subjugation of Humanity. Manifested in the combined magical prowess of all Mages within the Alliance, this place must fall for the Horde to conquer Lordaeron. Fortunately, Orgrim Doomhammer has saved his greatest weapon, ready to unleash it upon the unsuspecting Alliance at just this moment - Dragons. (Warcraft 2; mission briefing 1995)
Doomhammer, as the mission statement said, waited until the perfect moment to surprise the Alliance with his secret weapon, the dragons. Not only did it give the alliance less time to find a suitable counter, it also acts as a great weapon against the morale of the Alliance troops.
Source materials:
http:// wowpedia. org/Warcraft_II:_Tides_of_Darkness_manual#Gul.27dan
http:// wowpedia. org/Tides_of_Darkness_missions
http:// wowpedia. org/Retcon_speculation
Lord of the Clans, Christie Golden 2001
Rise of the Horde, Christie Golden 2006
Tides of Darkness, Aaron Rosenberg, 2007
Special thanks to Zugrah for helping me out.