Blood Magic is a tool, nothing more. It no more forces someone to be evil than a spoon forces someone to be fat.
Yes it can be used to harm, but to help as well. Blood magic can fuel a healing spell just as well as a harming spell. Noone will argue that it is a gift from a demon, or that many of its practitioners are wicked. But by itself, blood magic is neither good or evil, it merely gives you more options to deal with a situation. You could just as easily mind control that noble into adopting that orphan as kicking it. or not mind control at all.
You could argue that subverting the will of another is evil, but so is smashing someone's forehead with a hammer.
While it is not explored in game, I have to wonder if there are more benevolent applications of blood magic. If you can control a mind, can you also repair one that has gone through psychological trauma? If you can influence someone's blood, can you cure or build a resistance to blood borne diseases?
We don't know because the amount of research done with it tend to be almost entirely focused towards combat applications. To the Ebola comment, I'd liken it more to the use of a hammer rather than a disease since the disease will do its work on its own, but blood magic must be actively used for good or ill, only as evil as its user. Jowan can be a prime example, nevermind that he was told by Loghain to poison Eamon (under threat of death mind you, what do you think the templars would have done with him if Loghain hadn't intervened?) He dd attempt to help as best he could.
And if you want to take a look at Cullen and Niall, yes they've seen the worst blood magic can do, but that's no different than seeing a chainsaw massecre,yes its horrible, but its the person doing it that makes it horrible not the tool. if they hadn't used blood magic they would have found another way to cause that damage.
I guess the gist of what I'm trying to say is this Blood magic is dangerous, but is not by itself good or evil since it can be used for both. It just has a very bad stigma attached to it due to the deeds done by those that practice it.
That said, walking around with it is still akin to having a loaded gun in your hand, it'll make people scarred and get the authorities upset with you. =b