You could make a case for just about anything beyond that. It is possible to tank as blood, for instance. Is it the best possible? I'd make the case that it wasn't, I personally prefer going frost for tanking, but if someone comes along showing me an unholy spec, as long as they have the 5/5/5 of the first tier, I'd be willing to listen to their spec. There are mid-level tanking talents in all 3 trees (spell deflection in blood, frigid deathplate in frost, magic suppression in unholy) and with the exception of HC in frost, none of the upper level talents really affect tanking, other than threat generation.
Vampiric blood increases healing you receive from healers (from what I hear, I need to confirm this).
Unbreakable armor increases your parry and armor.
Bone armor reduces the damage you take.
All 3 trees have tanking components, and yes you can theoretically tank with all 3 trees. I'd make the case that frost made the best tank, unholy second, and blood a distant third, but you still can do it. The only thing you NEED as a tank spec is 5/5/5.