I kinda just jumped past the mana part.

If the Blood Mage worked that way I'd get so mad on BLM.
Even if the Mana Drain was much smaller, even any at all it'd screw BLM's rotation up lol.
If you're like 1 mana too low it'll be the difference between being able to cast your finisher or not.

It sounds like a disaster waiting to happen that only considers it from the Blood Mages perspective and doesn't take the whole picture into account.