What something appears to be is not necessarily what it is. There is no black without light. No light is you being blind because your eyes cannot function without light.
[Edited by Dhampy, 9/13/2009 5:41:36 PM]
Although to be practical, probably the only way to meaningfully define them is by comparison.
There is light in a pitch black room. The human body gives off ambient light and so does our clothing which is washed in phosphorescent chemicals.