Why are black boxes called 'black' while being orange in colour?

Black boxes store aircraft parameters and sounds, like pilot and copilot's conversations, radio communications and cockpit's ambient noise, and are crash-protected up to a certain level. The first development of black box in 1930s had a photographic film recording data in a light-less box, hence the name 'black' box. They're coloured orange to make them easier to spot amongst debris.