black art

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Black magic; witchcraft.

from the GNU version of the Collaborative International Dictionary of English.

• The art practiced by conjurers and witches; necromancy; conjuration; magic.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A practice such as witchcraft, sorcery, necromancy, or black magic.
• noun figuratively, by extension A process that is mysterious or difficult to master.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the belief in magical spells that harness occult forces or evil spirits to produce unnatural effects in the world