Color or Light
Black can be defined as the absence of visible light. For example collapsed stars, which due to their intense gravity can neither reflect nor emit light are called "black holes". Pigments that absorb light rather than reflect it back to your eye "look black." Conversely, the combination of all colors of light is called White.
In terms of pigment however, black is the combination of all (pigment) colors. If equal proportions of primary pigments are mixed, the result reflects little light and so is "black."
This can be explained as follows: the red pigment, for example, absorbs all light except red light; red light is reflected, and thus our eye sees the pigmented object as red. When many pigments are combined, whatever would have been reflected by one of the pigments is absorbed by the others. Thus no light escapes. (no visible light, that is; ultraviolet, for example, might still be reflected, unless some kind of "ultraviolet pigment" were added.)
Usage, symbolism, colloquial expressions
In western society black tends to have a negative connotation. In arguments things can be black or white, or shades of grey, the intensity used as an analogue for things such as truthfulness or right and wrong. (Note that when referring to the intensity of pigment or light, black is always the complete lack of intensity.)
Black is often used in painting and film to express the unknown or death.