As long as you spec one DK into unholy deep enough for Unholy Aura (increases party/raid movement speed by 15% at 2/2) then you wouldn't necessarily need to spec all of them for the 17 points required for the 20% individual movement speed increase from On A Pale Horse. Of course OAPH also decreases stun/fear duration by 20% so if PvP is your thing, then it would be a wise choice. Personally I never go over 41 points in Blood after getting 5/5 in Sudden Doom (insta cast, free Death Coils) and Heart Strike. Blood Gorged is OK, and DRW is more for soloing. Boxing DKs I don't think having the extra DPS from a DRW is going to be that much, unless ALL of them have it.

As far as a "tank spec", there isn't any one tree. I've tanked with my 74 DK at 41/5/17(ish) with the 5 in Toughness in Frost, the 5/5 in the parry talent in Unholy to get my 20% increased movement speed.