I don't think blood mages are underpowered, you are just not using their bonuses the right way. A gnome blood mage with mage plate and JJ (the only reliable god for mages right now) could get up to 36 MP at level 8 and and still have 6 potions remaining. It would be able to get 42 casts, and the loss of HP with each MP potion can be covered by HP potions and their ability to drink blood pools. 42 fireball casts would deal 1344 damage at lvl 8 and 1680 damage at lvl 10.
An elf blood mage could get 42 MP at level 10 with this strategy and have 1 remaining potion, then buy a keg of mana and get 4 potions. It would be possible to get 35 casts and chaos avatar, wich is useful for fighting enemies with high defense. 35 casts at lvl 10 would deal 1400 damage.
Blood mages are hard to level up with, because of their lack of resources to fight high level monsters. This is the advantage of using an elf