So far as i can tell the unique things they bring are:
Death Chain, Strangulate or Double Grip, Blood mirror and Tremble before me (3 sec fear on DND, 10 seconds till you can fear the same target again).
The only problem being is they seem squishy, they don't seem to survive a burst window very well especially when cc'd and with a few stacks on the flag.
Has anyone had any success with Blood in Rated Bg's? Maybe in a more aggressive setup?