Blood is primarily tanking, not DPS.

But going with 4x Blood, gets you a ton of Blood Worms which will provide decent healing.
You also get Rune Tap on each DK, which heals the main for so much and the party for so much; with four DK's doing this, it is close to 40% healing on all toons, as a 30 second ability.

Death Strike is also a healing move, which you can glyph and spec into.

Last tool is Taunt on round-robin.
Your guys will have 100% health each.
When one of them drops to 50%, hit the taunt key so the next one who is close to 100% takes threat.
Just bounce the threat around.

Blood will drop stuff at a decent pace.
Probably half the kill speed of Frost/Unholy (as in takes twice as long).
But you have a lot of passive healing and virtually cannot die, especially if you RR-Taunt and use Rune Tap often.