It is not black.
The man “credited with creating the prototype of the flight data recorder, or ‘black box,’” David Warren, has died. According to his obituary, his prototype was red. “Today’s black boxes … remain red or orange, to make them easier to find in wreckage,” after a plane crash. Why are they called “black boxes”? The obituary says:
How Mr. Warren’s red box came to be called a black box is not altogether clear. At the time, black box was a slang term in the Royal Air Force for a navigational instrument in an airplane. One story has it that a person who witnessed a demonstration said something like, “What a wonderful black box!”
Uh, good “story.”