I've had a good look around and not seen any explanation as to why black and red in particular are used, my guess (and it is a total guess) is that they were the colours that one particlar manufacturer used and it became a de facto standard.

The convention is that the red terminal is positive and the black negative. If you look closely you'll see often a little + sign on the red terminal somewhere. The colour coding is done to make it easier to match the terminals on the speakers to the amplifier. It is important to hook the speakers up consistently because otherwise one speaker may end up out of phase with another, and the electronics may be affected by a reversed connection.

As he mentioned, there is doesn't appear to be any official standard. It does however, apply to not only AV equipment. The red/black scheme has been in use in general electronics for quite a long time and appears to have become a bit of an unofficial standard.

It is just simple enough that practically anyone could look at the colors and assume that black is ground and red is hot.