In Origins you get it by selling a kid's soul to a demon.
That's pretty evil.
Actually, you can coerce the Desire Demon into nullifying the contract with Connor and leaving him alone, and intimidating her into providing the information to you in exchange for not destroying her.
It's also a specialization that's emphasized not to be inherently evil, so I'm as perplexed as Kain is about the statement. Duncan expresses that some Grey Warden mages turn to blood magic to give them an edge against the darkspawn, as a retort against the statement that it's evil. The order doesn't prohibit it, as The Warden can explain to Levi Dryden. The Joining and the phylacteries are types of blood magic. Finn uses a type of blood magic (with Ariane's blood) to locate the Eluvian in the Dragonbone Wastes. Merrill uses blood magic, and she isn't evil. I'm not seeing how it's supposed to be evil when we have multiple examples of how it's a school of magic that isn't limited to villainy or malevolence.
Even the lore reads: "Nothing inspires as much wild-eyed terror as the Blood Mage. Mages of this type take the raw energy of life and twist it to their own purposes. They can corrupt and control, and sustain their power by consuming the health of others, willing or not. The effects can be vile, but this specialization isn't limited to madmen and monsters. Many see it as the only form of magic that is truly free, because it's tied to the physical, not favors to spirits or demons."
That said, if the developers weren't able to establish reactivity towards being a blood mage (since the scene with Wynne was cut, and apostate Hawke being an illegal mage was mostly handwaved), I can understand why it was precluded as a specialization.