I solo play a DK and i got quite high exp and understanding of arena. I play blood for FCing in rated BGs, and frost in arena and soloing.
If it was the blood specc we had in Wrath when it actualy did deal some dps it would be imba to multibox i belive. In cate blood dont really deal dps, and the healing is kinda crap now compaired to what it used to be. The reason why blood wont work is the lack of dps, that lack will not give you the kill, you will start getting cced all over the place. While forst have some huge burst and can easy chop down a target in a few globals.