Black Magic has traditionally always been of the nefarious kind...harmful to others and only benefits one person...the curser.
But not even he/she gets what they want totally as due to the Boomerang effect of our intentions, if you put out a harmful intention, or manipulate a person for your own selfish motives, it usually comes back on the person doing the spell. That's why people who place curses end up sick or desperately unhappy and empty in their lives...the harmful energy they put out comes back to them, and it's usually x 3!
Vice versa if you set a helpful intention, such as sending healing to a loved one, then by the same law you recieve healing where you need it too. That's why the saying goes that the gift is in the giving.
It's generally a wise idea, given this fact, to always make sure your intentions are pure and beneficial to everyone...harmful intentions always rebound back to you eventually.