Blood works fine for tanking, I tanked every 25 man boss as blood, except for Sartharion w/3 drakes, who i had to go unholy for.
If you dont have an enhancement shaman, blood is really nice for raid buffing. It also has more HP and more single target threat generation than any other tree, as well as the most potential for self healing (mark of blood, vampiric blood, buffed death strike).
Not to mention next patch blood tanking is getting massive buffs, including a last stand ability, an ability that massively reduces burst damage, and a cleave.