black box

 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]

 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.