My take on the issue is that blood magic is the non-addictive alternative to lyrium potions. Jowan clearly learned his art from a book, so regardless of where blood magic lore originally comes from, you can learn it without bargaining with demons. Once you've done that, you have three options:
A) Continue to use the dangerous and addictive lyrium potions the chantry holds a legal (if not real) monopoly on, allowing them to control you.

Use your blood to fuel your magic.
C) Use the blood of others to fuel your magic.
Option A is stupid, and C is evil, but B? What issue could you reasonably have with B? Are you afraid that just because someone uses their own blood to fuel magic, it will inevitably lead to sacrificing children for your dark rituals? That's like if I searched through your browser history, and concluded you were going to eventually become a rapist.
In ancient Tevinter, they had no concept of "blood magic." There was just "magic, which, y'know, we mostly fueled with blood." Healers were blood mages. Abjurers were blood mages. Noncombat mages were blood mages. Everything was done with blood magic. The issue with the Tevinter was that most of what they did was really terrible anyway. They kept slaves, conquered nations, and did not care for human life. Because that was the sort of stuff they were up to, they kept blood slaves, used blood magic to destroy cities, and sacrificed humans and elves to further their goals.
But imagine a good Tevinter. A civilization with less overtly evil values. You go to the doctor, he cuts himself, casts "heal" and sends you on your way. At a construction site, you see a man covered in blood compacting the earth with magic from the primal school. A fire has broken out, and a man with a nail through his hand is causing a great mass of ice to fall from the sky, extinguishing the flames. Absolutely all of this is possible in the game. I played as a blood mage, in DA:O and DA][, and I never used blood sacrifice.