From LQWiki

[common] A collection of arcane, unpublished, and (by implication) mostly ad-hoc techniques developed for a particular application or systems area (compare black magic). VLSI design and compiler code optimization were (in their beginnings) considered classic examples of black art; as theory developed they became deep magic, and once standard textbooks had been written, became merely heavy wizardry. The huge proliferation of formal and informal channels for spreading around new computer-related technologies during the last twenty years has made both the term black art and what it describes less common than formerly. See also voodoo programming.

This article is based, in whole or in part, on entry or entries in the Jargon File.