blood magic on the passive tree is kindof terrible. Since it gives blood magic to ALL of your skills/auras. This basically makes aura's un-usuable.
so like, "hatred" which reserves 50% mana. will now reserve 50% of your life instead.
so you had 1000 life, 1000 mana, with 1000 life, 500man(500 mana reserved), you with bloodmagic would have 500life(500life reserved), 0 mana.
which admittedly, is the point of those passives past it. 50% less mana reserved. which would reduce the 50% reservation to 25%.
But still, not remotely enough to justify.
Conversely, ,that is where blood magic SUPPORT comes in. since with THAT, you can reserve 100% of your mana, with say 2 auras, or 1 aura, and 2 heralds, then placing Bloodmagic support on your active skills, they cost life instead. allowing you to use skills with no mana cost.