I was in 10DKs vs BRF heroic - the usual trick to these videos is that they take it in turns to tank allowing the other Death Knights to heal up from tanking and whatever incidental AoE damage is taken. The difficulty was usually dealing with certain mechanics that target melee weirdly, for example on Flamebender the fire lines would 1shot the entire raid if we didn't spread out.
If Blood DK didn't have enough healing to even heal itself up after a tank swap it would be fundamentally broken as a spec.