I forgot to explain the key thing, the ability is a toggle- it on default turns off once the blood mage is at 100% hp. Which means it’s parasitic yes, but only for a max of 10 second at most really [100% hp for 100% cost to mana]. Remember that black mages in ice 4 phase recover nearly 100% of their mana in one second. Even if you loose -10% mana a second, you still recover more mana then you loose.

Aka in combination of a black mage, and blood mage, the blood mage could heal in theory indefinite for 50 seconds if they are bellow 100% hp, which allows the healer to do more dps. This alone could shake up the meta considerably since it means you have

-3 dps, 1 healer, 1 tank. This would give black mage some more party utility. A perfect combination