We do not truly lack Block as Blood, however the order is different.
A Druid crits his target, procs Savage Defense on himself, and absorbs a little bit of the next blow.
A DK gets hit for damage, then uses Death Strike or Rune Tap to heal some or all of the damage back up.
Compared to Paladins and Warriors, yes, we do take more damage in 5mans. A "quality" shared with Druids, due to lack of a "proper" shield (which can reduce damage from each attacking enemy independantly). However it should not be a problem in 5mans, and in raidcontent it balanced out either way.
I would argue that depending on what you're taking the DK's selfhealing is much more powerful than Savage Defense at least since it's so controllable. It's frustrating to die before you get to selfheal but that means you could have done better. Not the RNG crit-based shield-proc.
-- (The Death of Rats, Terry Pratchett, Soul Music)