It's kind an old topic but here we go. In lvl 35+ dungeons your dark knight have Blood Price and Syphon Strike Combo to recover mp and it's your best friend.

Blood Weapon is useful for Off tank or go in dungeons below lvl 35 and its also good carry some Hi-Ether and Hi-elixir to recover a bit of mp, in case you need.

I never use blood weapon as main tank and never have mp issues on dark knight to keep darkside up and spam unleash. Even when the fight take so long due bad dps and my drk lose the effect of darkside i keep using Syphon Strike Combo and Blood Price if its ready to use, it's enough to recover a great amount of mp and turn on darkside again.

I've seem lots of garbage drk dropping grit to use Blood Weapon, they lose the aggro for dps and healer and things get worse.