It doesn't have to be evil on principle. In practice, even the Tevinter Imperium discourages it (although with great hypocrisy). The Grey Wardens aren't a good example since even they admit that they will use whatever it takes to defeat the darkspawn. After all, nothing can be worse than the triumph of a Blight. However, blood magic is tied to demons teaching it, to mind control, to desperate or terrorist mages, to human/elven sacrifice, etc. No wonder is seen in a bad light, and not just because of Chantry propaganda (the Dalish are against it too).
I'm not arguing that it can't be misused, I'm addressing that we have multiple examples where the school of magic is used for purposes that aren't villainous. Grey Wardens fighting darkspawn, or apostates trying to stay alive against templars who can nullify their powers. It's hard to say it's inherently evil when we have two games where it was used by people who weren't malevolent or villainous. Pointing out that there are reasons why it's distrusted by some people and groups doesn't really change that, as it wasn't the focus of my statement.
Don't forget the last part: "It remains an undeniably violent and self-destructive discipline, however, and the Blood Mage must be careful. The temptation to take just a little more is always there."
Considering it involves the extraction of blood, I don't think anyone denies it's violent; we also see blood mage characters resist crossing certain lines and losing themselves, so again, I don't see how it's evil.