The Collaborative International Dictionary of English v.0.48:

Black box \Black" box`\ (bl[a^]k"b[o^]ks`), n.
   1. any electronic instrument or part of an instrument whose
      function is defined, but which is treated as a unit
      without consideration of the internal mechanisms; broadly,
      any device whose internal workings are considered as
      incomprehensible or mysterious by the user; as, to treat
      the meter as a black box and take its readings on faith.
      [PJC]

   2. (aviation) a device which maintains a record of cockpit
      conversations and the readings of many of the instruments
      on board an aircraft, continuously or at frequent time
      intervals during a flight. It is of rugged design to
      withstand a violent crash, and is used to determine the
      causes of aircraft accidents.

   Note: They are often brightly colored to assist recovery, not
         actually black.
         [PJC]

WordNet (r) 3.0 (2006):

black box
    n 1: equipment that records information about the performance of
         an aircraft during flight

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

black box

    An abstraction of a device or system in which only
   its externally visible behaviour is considered and not its
   implementation or "inner workings".

   See also functional testing.

   (1997-07-03)