Like I said - I was until pretty recently under the impression that blood magic was pretty neutral but it seems according to Bioware it's not supposed to be. Do I understand exactly why? No, I honestly don't. A couple things stick out at me though:
1) The World of Thedas specifically says that the more violent the pain or sacrifice, the more powerful the blood magic. Note that this is not the same as simply more blood. That suggests that the power of blood magic is not actually just the manipulation of blood, but how "bad" the act is.
2) The World of Thedas specifically says that blood magic makes the mage more susceptible to demons. (I would argue that using the word susceptible instead of more alluring indicates that something about blood magic lowers a mage's ability to refuse a demon rather than just making the mage more enticing.)
3) There have been at least hints that blood magic may weaken the veil.
I think Bioware wants to make blood magic evil and corrupting. I honestly don't think they've done a very good job of that in either dragon age games. Even in DAO, I chalked up characters reactions to be more because someone assumed blood magic was evil, not that it actually was evil.
Perhaps eventually they'll be able to introduce blood magic in such a way that players can get a better grasp of what blood magic entails. While I would love to see that, I can understand why they wouldn't want to spend an extensive amount of time on a storyline that would affect only a small percentage of players. They're already going to have to do a lot of work for the different races in DA:I.