There are plenty of theories that seek to explain the term "black box" but the truth is nobody knows for sure.
The term first popped up at a meeting of Britain's Aeronautical Research Council in 1958. That was several years after Australian chemist Dr David Warren began work on the precursor of today's flight recorder, a device he called the ARL Flight Memory Unit.
Warren was a member of a team trying to establish why a De Havilland Comet broke up in mid-air over India. He concluded that a cockpit voice recorder would help provide answers, and on that basis he developed the ARL Flight Memory Unit to record cockpit conversations and instrument readings.
On a modern aircraft, that data is divided between a Flight Data Recorder and a Cockpit Voice Recorder – but neither they nor Warren's original ARL Flight Memory Unit have ever been black. They're bright orange, to assist in their recovery after an accident.
In the aftermath of the mysterious disappearance of flight MH370, it has been proposed that as well as onboard recorders, flight data be continually streamed from the aircraft to a ground-based station.