Well dks are tanks that take more dmg by default compared to the other tanks as they rely for a big part on self healing cq building up a blood shield. But when you take 2 consecutive big hits then it might indeed feel that you gonna die, definitely with a resto druid hots on you. But in fact you are probably just fine. Either way I'd say that resto druid paired with a blood dk demands way more active cd management on both toons compared to any other tank with any other healer.