Blood is the ‘‘tank’’ specialization, you have alot of self sustain, meanwhile ‘‘Frost Presence’’ is the tanking presence that you should have on while tanking to take less damage.
All DK specs are able to tank as long as you are in Frost Presence, but for high end content you better go Blood Spec for the self sustain.
Dual Wield for Frost Spec (there is also a Dial Wield talent in Frost spec that gives you more damage with off-hand weapon).
Blood and Unholy are generally 2h’der specs, there have been some talks of Dual Wield Unholy but that is something you have to look into yourself, as I am not familiar with the data behind it.
Weapons as Blood are more or less statsticks. If the threat and survivability is there, then you can start looking at what damage you might be able to contribute with and how (which goes without saying is a bigger or smaller factor depending on raid size).
I did i dungeon run and they got mad at me because i was frost spec and had frost presence activated, they said it gives threat so learned it the hard way, i was dps
So say i wanna go Blood spec with 2H Axe/sword as DPS in dungeons and maybe some raids later on, this is gonna be ok and people not expecting me to be tank instead?
For solo i also prefer Blood, i like that i get fully healed while questing and not die (so much)
Have You read any of the wowhead guides about Death Knights? They are usually well written, and will give You both general overview of the class and specifics about every spec.
In case You missed it, You can start here: https://www.wowhead.com/wotlk/guides/death-knight-class-overview-races-tier-sets-trainers