Why is the "black box" on an airplane not black?

Because in the event of a crash, a bright orange box is a lot easier to find than a black one when searching through wreckage and debris. So it wouldn’t be quite practical for it to be black.

I have heard that it’s called a black box because it is charred black in a wreck.

Black box is a common term for something that you works without letting you see how it works. Or where you don’t care to see the workings, it gives the desired results, and how it gives the results is not a your concern.

Say you make airplaines – your leave make a place for the black box, and provide hookups so it can measure airspeed and altitude and which controls the crew is using and whatever else it measures.

Somebody else builds the black box. And they may completely change its internal design without telling you, but you don’t care – you provide the connections and they can do whatever they want with their “black box”.

Up until 1965 the FDRs were in fact black boxes until someone had the ‘bright’ idea of making them easier to find.
The old name just stuck.

http://aviation-safety.net/investigation/cvr/links.php