I wish for it to come back, although it presents a problem with the current way health is managed... If you can't regenerate health the specialization would either require some sort of special healing or spells with massive damage that make worth all the health you're spending.
Although I agree that it should be kept very hard to get, like in Origins it can only be obtained by making the deal with the Desire demon, it wouldn't make sense for the knowledge to be found just about anywhere.
Also, it is not blood magic until you use someone else's blood, so there shouldn't be any stigma to it. xD
I don't think it should be hard to get, honestly. Because that will translate into a 3 hour fetch quest. Just let me learn the spec from a desire demon (aka, short quest), or from a companion w/ a blood mage spec.
I've thought a lot about blood magic with the current healing system and i don't see why it would present a problem. IN FACT it should be easier than ever to implement because there are no healing spells (except resurgance). Spells like hemorrhage would sap a certain percentage of your health, but might not be capable of activating underneath a certain health threshold, like mana. Also spells like DA: II's grave robber would be perfect for this game, with a bit of damage resistance thrown in to make it balanced.
Plus imagine the focus ability. I picture an ability that hemorrhages (damages) enemies in a large AOE and transfers that into health regen or damage resistance.
EDIT: In terms of the current battle system, lemme put it like this:
-We can already "assume control" over enemies with confusion grenade.
-There are no healing spells to interfere with blood magic mechanics.
-Heal on hit and heal on kill are already cornerstone to this system, perfect for blood magic
-Death siphon more or less acts the same way
-Reaver already looses health with certain abilities, and regains it with others (like rampage)
-Mana could still be used by a blood mage in this combat system by using non-blood magic spells
-Blood magic itself would cost a portion of health in exchange for massive damage. It would have to be a big fraction of health to cast, to make it more fair.