The only caveat about using white instead of grey, by the way, is that there are two ways to make somehting white: correcting the white balance, and blowing out the highlights. You can effectively turn anything white if you expose it long enough. Grey can only be grey if the white balance is correct.
Black, on the other hand, can be black no matter what. Take a piece of red paper, a blue one, and so forth into a dark room with no lights. What color will they all be? Black, of course. Stay away from black objects for white balancing.