If you add black to the white it will inevitably end up mixed into a black/white. I don't prefer this look, so my vote goes against adding it to the top. Not to mention I thought that was a no-no? Doesn't it trap the detritus and life in the sand bed which can cause an ammonia spike the bio may not be able to handle? While you certainly can make any changes you want, this is one of those things best decided before starting a tank.