I do not believe any school of magic to be inherently dark, but blood magic is considered one of the more dangerous disciplines, and for most users that I've met, its use was born out of fear and desperation. Sadly, it leaves users of blood magic even more susceptible to possession. If you find an abomination, they once were a mage. Which makes it all so very sad.
I think it's best I keep to my battle magic and healer disciplines.