Why is a plane's Flight Data Recorder (sometimes referred to as an Accident Data Recorder) called a 'Black Box' when it is not black?

Why is it called a black box?
Although commonly referred to as a 'black box', the device's official name is flight data recorder (FDR) or cockpit voice recorder (CVR).

Originally, flight recorders consisted of magnetic tape encased in a fireproof box that was painted black to protect the metal and prevent rust, hence the terminology 'black box' - an expression that was made popular by the British during World War II.

However, after FDRs were mandated by the aviation industry in the 1960s, regulations stipulated that all flight recorders must be painted in 'international orange' - making them highly visible and easy to distinguish in the event of an incident.