The tank is gaining Vengeance, because they're being hit by the mob.
This increases her attack power, by a fair amount.
So the tank can hit much harder than if the mobs were ignoring her.
This is a threat mechanic, so the tank holds aggro in a raid or group.

If the tank isn't taking a lot of hits... which would be the case with multiple Blood DK's...
Then they won't have much Vengeance, so will be hitting for a lot less.

The Blood DK's do have significantly higher survival than the other specs.
Which means, as long as there isn't a time limit on your dungeon run, and as long as the boss won't Enrage after 10 minutes or whatever... multiple Blood is an option.
It will take a lot longer to clear the instance, than running one tank and the others as DPS.

Quite a few of the 4x DK + 1x Healer teams went with 4x Blood DK's, in PvP gear.
Back in 4.0.3 and 4.1, when we didn't have the gearing to outgear the instances.
Most of the heroics were doable, eventually.
And all of the normals were, I believe.
The movies would still be in the movie section, on this site.