Magic attempted for evil purpose, calling upon evil spirits or the Devil.
Dean: So Roy's using black magic to bind the reaper?
Sam: Ok. uh...If Roy's using some kind of black spell on the reaper, we gotta...figure out what it is. And how to break it.
Sam: Yeah. You gotta build a black alter with seriously dark stuff. Bones, human blood. To cross a line like that, a preachers wife. Black magic. Murder. Evil.
Black magic is use for evil purposes and can be used by anyone who knows about it (it is more commonly used by witches and demons though).