I hadn't thought of the possibility of getting Unholy Blight as a Blood tank. Even if you have to make some sacrifices, it might still be worth it for an AoE tanking perspective. I am pretty sure your single target threat will suffer, however.
Something like this for example
45/5/21
You lose:
- 10% damage when above 75% HP
- However much ArP BG will be
- 30% crit bonus on Blood Boil, Blood Strike, Obliterate, Death Strike and Heart Strike
- The possibility to get Annihilation so you can use Obliterate rather than Death Strike
You gain:
- Morbidity (depending how you spec at the moment)
- Epidemic (depending how you spec at the moment)
- Either 5/5 Necrosis, 3/3 Outbreak & 2/5 Necrosis or any combination of points in that tier.
- 2% Str & Parry from Shadow of Death
- 20% stun/fear reduction (optional, flavor perhaps)
- Unholy Blight
Maybe it will be worth it. Maybe not. Also first time I hear that Blood isn't GCD starved, but I haven't been Blood DPS for a long time I guess.