I consider blood magic to be a lot drugs of our world. When you first try it, with only a little and with the classic non harming stuff, you're most likely gonna have a great time but when it gets bad, it gets bad.
I can see why those would resort to blood magic and I see no issue when using it like merrill did to repair the mirror besides the demon stuff. It is a tool, fine when your using your own blood and only for constructive purposes but when you begin sacrificing innocent lives and summoning demons, that is when it is too far,
Just as Solas said, it is a tool and is fine as long as you don't depend on it too much or become reliant on it. To be honest, blood magic is no different that using lyrium, lyruim is literally the blood of the titans. So no, it's no inherently bad, it is the people that use it for negative reasons that are bad.