The issue of Blood Magic is one of the more uncertain areas of Dragon Age and something I've always enjoyed because it's the perfect sort of tool for gamers who enjoy a little dash to to their characters, particularly mages. Dragon Age is, fundamentally, about the greys in conflicts of good and evil as the first and primary story of the franchise is the GREY Wardens versus the Darkspawn. Also, the fact the Warden can be completely evil but still be a hero as long as he stops the Darkspawn.
One of the appeals I've found to Blood Magic is that it is completely reviled in setting. To be a mage is to be considered suspicious and a ticking time-bomb but to be a Blood Mage is the equivalent to admitting you're the professional Baby-Eater at the Imperial Court. Blood Mages occupy a place akin to the Sith in the Dragon Age setting and it's not because of their idealogy. It's because they have become intimately associated with being an "evil" mage.
The thing is, the games go out of their way to make it clear Blood Magic is not intristically tied to evil. You don't have to sell your soul to the demon in the Fade to learn Blood Magic. You can just SCARE it into teaching you Blood Magic. Jowan learned it from a book. You can learn it from a book in Awakenings. There's more, too. The Grey Wardens and Templars both use Blood Magic in-game with the hypocrisy of it explictly noted. Gamers are left with a sense that Blood Magic is inherently dangerous but perhaps unfairly reviled.
Except, of course, that those who walk away with this impression also are ignoring other factors. Broken Circle is a complete cluster**** of epic proportions. Blood Magic was used to help "liberate" the Circle from the Templars but it has turned into an epic disaster as the Blood Mages have found, instead of liberation, death at the hands of demons. You can even spare one of the Blood Mages who is not repentant about her rebellion but simply how it turned out. There's also the ambiguity of Connor. Connor didn't get possessed because he's a mage. He explicitly used Jowan's magic to summon a demon in order to make a pact with it. Jowan is a Blood Mage.
It's easy to assume Blood Magic = Demonology.
But it's not.
The thing is, I do think Blood Magic's reputation isn't UNFAIR either. Blood Magic doesn't make you evil but it's the kind of thing which appeals to evil-doers. People seek out Blood Magic despite its reputation because they are willing to do anything in order to achieve their aims. The kind of people we encounter who are Blood Mages aren't those who are studying it because they believe it is an unfairly maligned discipline (Avernus is the closest we encounter to someone like that and he's utterly ruthless in the pursuit of science) but because they believe it's reputation implies power is to be had with it.
Power which the sociopathic will gravitate to because most people who aren't sociopaths wouldn't want to learn it.
Blood Magic may not NECESSARILY come from demons but it's also the case that demons certainly teach it. We get it indirectly confirmed in First Flight and directly confirmed by the author that Blood Magic taught by demons is deliberately incomplete. They're not trying to directly trap the souls of human beings but any human who thinks they know everything about Blood Magic who learns it from a demon is a moron. It's much-much more complicated than it appears.
I'm inclined to think that Tevinter is an example also of what "non-evil" Blood Magic's pitfalls are too as the Tevinter aren't overrun with demons but are implied to be overrun with Blood Magic. I think this is due to the fact Blood Magic still can be abused as the slave culture and culture of Darwinist meritocracy brings out the worst elements in mages anyway.