The Collaborative International Dictionary of English v.0.48:

Black art \Black" art`\
   The art practiced by conjurers and witches; necromancy;
   conjuration; magic.
   [1913 Webster]

   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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

black art
    n 1: the belief in magical spells that harness occult forces or
         evil spirits to produce unnatural effects in the world
         [syn: sorcery, black magic, black art, necromancy]

The Jargon File (version 4.4.7, 29 Dec 2003):

black art
 n.

    [common] A collection of arcane, unpublished, and (by implication) mostly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.


The Free On-line Dictionary of Computing (30 December 2018):

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   A collection of arcane, unpublished, and (by implication)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]