My science teacher described it best; Red, blue and green are colours, yes? They reflect light in a certain wavelength and that's percived by our eyes/brains as different colours. The way he saw it, black is not a colour, it's a shade. Unlike red, blue and green, the light isn't reflected back by black at all - it's absorbed (hence black being a 'warmer' colour). So whereas colours (red, blue and green) are areas where a certain material is reflecting light, black is an area where a material is not reflecting light (therefore is an absence of light). As 'space' in an area where there are no materials, there can be no reflection of light thus, blackness. Hope that wasn't too confusing or hard to follow.