Wordorigins.org

When an airplane crashes, what follows is inevitably a search for the black box, or more accurately the two black boxes, one that records the voice conversations in the cockpit and the other that records data about the flight, such as location, speed, and altitude. The odd thing is that whenever the boxes are recovered and shown on the news, they are not black at all. Rather, they are painted bright orange for visibility at a crash site.

So why are they called black? Black box is a generic term for a piece of electronic equipment on an aircraft. The term originated in air force slang during World War II. The first black boxes were radar bomb “sights.” Eric Partridge’s Dictionary of R.A.F. Slang (1945) contains the following entry:

Black box or gen box, instrument that enables bomb-aimer to see through clouds or in the dark.

The term was also used by US air forces during the war. Hamann’s Air War (1945) defines black box as radar.

Later, the term expanded to include various electronic navigational devices. When the flight recorders started being installed on civilian aircraft in 1958, the name was applied to these devices. The original WWII black boxes were literally colored black and many pieces of avionics equipment still come in black housings, but the term is applied to all of them regardless of color.

There is another type of black box that also takes its name from these WWII devices. A black box can be a mechanism whose internal workings are not understood, but its function is. If an engineer knows that the device will give output Y if he inputs X but doesn’t understand why, then that is a black box. The Journal of the British Interplanetary Society records this from 1953:

As far as the layman is concerned, a phantastron is a “black box” which will divide the frequency of its output pulses by any integral number between 2 and 20.

This sense is from the fact that aircrews did not understand how their black boxes worked (the components and processes were closely guarded military secrets), they just knew they did.

(Sources: Oxford English Dictionary, 2nd Edition; Historical Dictionary of American Slang)