Black magic or dark magic has traditionally referred to the use of supernatural powers or magic for evil and selfish purposes.Voodoo, too, has been associated with modern "black magic"; drawn together in popular culture and fiction. However, while hexing or cursing may be accepted black magic practices, Voodoo has its own distinct history and traditions that have little to do with the traditions of modern witchcraft that developed with European practitioners
Basically it is the use of supernatural powers to curse some one or any evil purpose.When all other tactics fail, you can use black magic to bring about the outcome you desire. Black magic is fueled by powerful forces and spirits, so it's important to know what you're getting into before you start casting spells or performing hexes - otherwise you might be the one who gets harmed.
Black magic is considered a dark art because you use it to get what you want by means of controlling someone else.