Add a talent extremely deep in Blood. Call it Taint Blood...

Taint Blood - Your Heart Strikes taint the victims blood lowering all healing over time effects by 50% effectiveness, and lowering all absorbtion affects by 50%.

This would give the spec what it needs to compete in pvp. Currently Blood is terrible, and has no ability to pressure at all. The spec is basically a arms warrior with no mobility, and no mortal strike.

A 95% physical damage class can not be affective without a anti-healing mechanic. Blood DKs deal about as much magical damage as a rogue does with bleeds/poisons. Having all physical attacks mean you have no ability to pressure or even threaten a healer without mortal strike.

Imagine if Warriors didn't have mortal strike they would have no ability to kill anything. This is the situation that Blood DKs are in. They have horrible mobility and very low burst potential much like a warrior because all there damage is mitigated by armor except for Death coil. On the flip side a Blood DK doesn't have the armor pen of a warrior so they are extra gimp.

Giving them a solid anti-healing mechanic could bring the class out of complete failure.

Currently there are a total of 11 Blood Death Knights over 2000, and only 2 of those are over 2200. It has the lowest representation of any spec in the game. Even prot paladins/warriors have more people have 2k and 2200.

Toss this spec a bone....