Magic attempted for evil purpose, calling upon evil spirits or the Devil.
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).
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.
Sam: Yep. So, let's say somewhere down the line, Walter learned some pretty black magic.
Dean: Yeah. And let's say he's pissed at these people for wrecking his movie.
Sam: Yeah, there are bird bones, rabbit's teeth. This cloth is probably cut from something Janet Dutton owned.
Dean: So we're thinking witch?
Sam: Uh, yeah, and not some new age wiccan water douser either. This is Old World black magic Dean, I mean, warts and all.
Dean: The serpent is Tiamat, which is the, uh, Babylonian god of primordial chaos. I guess their, uh, priests were working some serious black magic.
Judah: Well, six. If by "friends" you mean co-worshippers in a black magic cult. They were getting together that night to perform a ritual.
Charlie: According to the notes I found, it’s been owned and used by cults, covens, even the Vatican had it for a while. There’s a spell inside that thing for everything. We're talking some black mass, dark magic, end-of-times nastiness.