Black magic is any form of magic performed solely for the benefit of the practitioner, often to the detriment of others. Traditionally it was any form of evil magic, though this definition is more general.
Conventionally and historically speaking, black magic is a term that loosely refers to magic used with evil or malicious intent, usually performed with the aid of the Devil, and involving sacrilegious or criminal acts such as blasphemy, animal or human sacrifice, desecration of the dead, etc. In this sense, it has often been used as a synonym of the term witchcraft (not in the neo-pagan sense, but rather in the conventional sense of nasty crones stealing babies and consorting with Lucifer).
Probably due to its traditional association with devil worship or demonology, black magic is promoted among self-labeled left-hand path religions such as the Church of Satan and the Temple of Set, and a large number of chaos magicians. In this tradition of Western occultism, black magic has taken on a distinctly new meaning. It is most commonly viewed as a tool for achieving personal liberation, and for effecting change in the world according to one's individual will, usually unrestrained by any conventional codes of morality or ethics. It appeals to the personal autonomy, accountability, and responsibility of the magician rather than to any externally imposed moral or ethical order.