Elton John is dead wrote...
You say it's neutral?
No. No matter what it's used for it costs the life of someone whether it be another person or the user. Eventually the user (if they desire more power) may seek to obtain the most powerful of spells which would result in the end of their life wherein the demon will take full control to wreak havoc in the physical realm.
First off, there's absolutely no evidence to support the assertion that blood magic leads to an increase in Abominations simply by virtue of
being a blood mage. If anything, there are only two ways being a blood mage would cause an Abomination to appear:
1) Arrogance and overconfidence cloud the Mage's judgement, wherein a Demon preys upon a moment of weakness and seizes it -- a theory I've tossed around before, but that deals more with the guile of a Demon and the mental fortitude and sense of the Mage then it does with blood magic itself.
2) Using a particular spell that causes the Veil to be torn, and then having a Demon come around full circle on you and forcibly possess you, making you share a mental apartment with it. The only upside is that you can declare him a dependent on your tax forms.
Secondly, if a Mage desires more power -- which can be seen in Mages that abstain from blood magic as well -- that's a mark against the Mage.
Not the magic itself.
Since blood magic is of the demons and demons are evil, that makes blood magic itself evil in nature.
Blood magic is learned from Demons because they're the quickest source to learn it,
not the only source. The arcane arts are eternal in the Fade and to its denizens -- as Torpor will state -- but there's sufficient evidence and alternate theories proposed in-game to suggest that one can learn blood magic elsewhere.
For example, history suggests and states that blood magic was taught to the Magisters from Dumat, the Dragon of Silence. Alternate theories exist that state the Elves of Arlathan knew it and taught it to the Magisters -- supported by how Merrill tells Master Ilen that the clan shouldn't be afraid of "the old ways", though whether she was specifically referring to blood magic is unknown.
In addition, Jowan learned blood magic from books. The Warden-Commander of Amaranthine can tell the Baroness -- whom he/she believes to simply be a powerful Mage, and as far as anyone knew that's all she was until later developments.
That suggests that you can learn blood magic
from a Mage,
not a Demon.
Next, the Chantry is known to have stamped out blood magic's practice. It's a given that they probably burned any and all tomes they could find on the subject that would allow one to learn it safely, thus leading to Demons being the quickest avenue to learn it. This doesn't mean they burned all the books in existence. Simply all those they could find.
If we were to believe the book claiming Andraste was a Mage was burned by the Chantry -- likely given the views in it that run contrary to Chantry dogma -- then book-burning would be well within their scope of things they'd do.
Finally, if we were to take the cut Wynne confrontation moment as canon despite the bugs it caused and the fact that it was scrapped for those reasons, the Wardens teach their Mages blood magic.