the white and black sand mix i am told actually is a new and rather athetically pleasing look. or if the sand bed is shallow you could just pile the black on top of the white and have a deep sand bed, just be carful not to mix it.
Entropy is right, black sand does get ugly. i had black sand in a tank and how i dont know but it got dirty looking. it goes against logic, we usually buy black so it doesnt show dirt but the black sand turns grey dusty etc.
Headless donkey how do you have blue sand?? are you using that colored sand sold for reptiles?? if so i was told by the company that is not tank safe, least not in salt water. the dyes leech out. it isnt anything more than food dye but still.. if you have another source please share, i have never seen colored sand tank safe. I would love to set one of those up!