KMD and 3holelover, the term "black box" went further back than the "black finish", I believe. In electronics one of the users was a hypothetical box with an unknown function and unknown contents. All you had were the various gozintas and gozoutas on the box to manipulate and discover what was inside. On the other hand it MAY have started with the original WW-II military equipment that was typically black for airborne Army equipment. But Navy equipment tended to be a sort of light gray while Army radios tended to be olive drab. ALL tended to be referred to as black boxes from time to time.