Absorbing all light and returning none. The absence of what would let a thing be seen.
Old English blaec (black, dark), from Proto-Germanic *blakaz, from a root meaning to burn or scorch — the same deep root, by an odd turn, that gives 'bleach' and 'blank,' words for whiteness, through the sense of a burned, ash-pale surface.
The colour that returns nothing — what a thing looks like when there is no light left in it to answer with.