The Blood Dks niche is probably going to be his DPS while tanking because the entire Blood Dk tool kit falls short of the baseline warrior ability Ignore pain.

then they have

Demo shout (20% magic/melee DR) 1 min CD
last stand ( 30% increased HP) 4 min CD
Shield Wall (40% magic/melee DR) 3 min CD
Spell reflect (30% magic DR) 25 sec CD

Everyone playing a Prot Warrior will litterally tell you "I feel invincible right now" i have cooldown for any situation and my spammable AM is basicly a blood shield thats 7 times more powerful that works against magic dmg as well.