black art: n. A collection of arcane, unpublished, and (by implication) mostly ad-hoc techniques developed for a particular application or systems area (compare {black magic}).

:black art: n. A collection of arcane, unpublished, and (by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}.
-- The AI Hackers Dictionary