Source : Webster's Revised Unabridged Dictionary (1913)

Black art \Black" art`\
   The art practiced by conjurers and witches; necromancy;
   conjuration; magic.

   Note: This name was given in the Middle Ages to necromancy,
         under the idea that the latter term was derived from
         niger black, instead of nekro`s, a dead person, and
         mantei`a, divination. --Wright.

Source : WordNet®

black art
     n : the belief in magical spells that harness occult forces or
         evil spirits to produce unnatural effects in the world
         [syn: {sorcery}, {black magic}, {necromancy}]

Source : Free On-Line Dictionary of Computing

black art
     
        A collection of arcane, unpublished, and (by implication)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 optimisation 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}.
     
        [{Jargon File}]