The problem with saying 'I don't like 'X' because 'X' is evil' is a logical fallacy, nothing is inherently evil, it's all subjective. The Desire Demon who gives you Blood Magic in DAO says she has a contract with the child and they made a deal. In its mind, the demon probably thinks what it's doing is right. Morality is a man-made thing and totally subjective. I personally don't think the demon is right for example, because I wouldn't want something like that happening to me because that would be harmful to me and effect my quality of life. Someone else however, might totally want a desire demon to possess their bodies.
So Blood Magic ISN'T evil 'because it is'. Blood Magic like any other type of magic in the series can be used for good or evil. A blood mage (like my Warden and Hakwe) would use it to save peoples' lives or stop a blight. A regular mage might use a normal fireball to burn newborn babies to a crips. My current warden is a dalish elf, Blood Mage/Sprirt healer who is incredibly sweet, nice and helpful to others. Same as my Hawke who always uses Nice/Diplomatic choices and do I even need to point out Merrill? Freaking Merrill, she's the most sweetest heart in the entire series.
Not to mention that Warden mages use blood magic and aren't restricted in its use. Then there is the practical benefits of blood magic such as extending a person's life. Avernus in Soldier's Peer is centuries old because of the use of blood magic AND has been using it to avoid the calling and corruption of the taint (think Larius from Legacy).
If you want to talk about the risk of demonic possession because of Blood Magic then first go read the information about Spirit Healer. "It should also be noted that the calling of a spirit healer is a dangerous one. Contacting anything beyond the Veil inevitably draws the notice of demons, sometimes very powerful ones. More than one tale exists of a spirit healer being fooled by a demon masquerading as a benevolent spirit, and inadvertently bringing them across the Veil... or being tricked into letting down their guard, and possessed. As such, once a mage becomes a Spirit Healer they must especially heighten their vigilance for the remainder of their lives. It is a calling that not all will gladly suffer"
Blood Magic isn't evil in nature, just something that can be used to harm others if the mage wants to, just like any other magic. You can try to argue that It's like the Dark Side of the force. Maybe it calls the user to do things to harm others but all you need to be is smart and aware. Even then however there are pleanty of Jedi and Grey Jedi which used dark side powers to do good without ever becoming corrupted. I don't know if Bioware has actually come out and stated that Blood Magic is evil, but quite frankly they would be wrong if they did.
As for the OP, he just sounds like one of those 'I don't like this so therefore nobody can have it' people.