Black boxes which usually hold the clues to a plane's crash, are built strong enough to survive bomb blasts, violent impacts and intense fires. The black box records voice and flight data. The cockpit voice recorder continuously collects data. They are actually orange in color and a very high quality heat resistant paint is used, so that they can be spotted easily even at dimly lit crash sites. They are always placed in the tail end of the flight.
The term 'black box' is generally used to describe a thing/process which is not well understood by most people, but which produces something useful or well known. The term black box is used for the data recorder on airplane, in this context for similar reaso: the crash is the mystery process recorded by the black box - once you open up the black box, the process can be understood.