Dave, I don't know what you have on your walls, but Durock (brand name for the cement board manufactured by US Gypsum) is not black - it's grey with a cement- like texture. There is a panel made by USG for exterior stucco backing that is black; it looks basicly like black drywall. It's not really intended for tile, so who knows what it will do. If you have white grout, I could see how there might be color variation where the grout differs in density (a darker color might work better) but then it could be some reaction with the board surface. If by separation at the corners, you mean cracked grout, the fix is not to grout inside corners. Fill them with a matching caulk (this is good practice in any case). I think the panel will need to be positivly IDed before any real conclusion can be drawn. Even then, if it is an unconventional backer such as mentioned above, it may be hard to predict how it will perform, especially with respect to it's water shedding capability. Normally, a water proofing membrane is applied along with cement board. The stucco backer can shed some water, but ... See what you can find out and post back.