To be honest, none of the legendaries for Blood DK are gamebreaking in terms of power increase. Comparing to something like the Barkskin legendary for Guardian druids; Barkskin (40sec CD with talents) reduces dmg taken by 30% up from 20%, and grants a free Ironfur (1K+ armor) for 7 sec + 32% HP over 3sec heal at the beginning and end. It’s the equivalent of Vampiric Blood proccing a free death strike + rune tap at the beginning and end of the CD…
I’ve used Brynador, Superstrain, Phearomones and Death’s Certainty. Comparing them to each other I didn’t have any runs where one of them felt superior to the rest.
Superstrain is the overall “safe” pick since the current meta above +10 involves constant kiting and you get around 12-15% more dps from the 2 extra diseases. In addition you get the random 5RP procs from Frost Fever, which is just free resources while you are kiting so you can pool RP for Death Strikes. It’s probably why the few Blood DKs actually doing high keys are using it.
Phearomones is hard to measure in actual numbers outside of the 8% haste, but constantly disrupting mobs with the fear proc is very useful for reducing overall damage in dungeons with a lot of casters/ranged mobs. It’s also my favorite pick for Torghast (until you can pick up the effect as an anima power).
Brynador feels more useful in situations where you can’t kite as much, and it has great synergy with Red Thirst (since it refunds RP after you Death Strike, rather than just making it 15% cheaper up front).
Death’s Certainty is mostly a ST dmg increase. It’s fine for Tyrannical bosses and probably raiding, but if you’re looking for damage Superstrain scales better with more targets and also provides some defensive benefits from the 5RP ticks.