I wish there was a list somewhere of all the blood mages in DA2, because I also remember a lot of them just turned to blood magic in self defense, or were slavers from Tevinter. It's true that Bioware tried to "both sides" the issue pretty hard but I thought it was clear nonetheless that Meredith and Elthina were to blame for the whole mess.
Be that as it may, my point was that Cullen didn't really change that much in Inquisition. If we assume that the templars (and Cullen) were justified in DA2, then Cullen didn't really need to "come around", and he kinda didn't.
This thread is getting pretty off topic, we really need that OT lol.
Every mage in DA2 turns to blood magic, EVERY mage. I honestly can't think of a single mage you interact with who doesn't. They really tried to "both sides" the dispute really hard with DA2.