IMO, black sand looks beautiful in the aquarium. The only problem I would see, Im not even sure if this is true, but it would be that the black sand would absorb alot of the available light in the tank. This could turn into a problem if you were to use higher intensity lights because it would increase the temperature and might harm the CUC if they decide to burry themselves and even if not, it would still absorb alot of the light in the tank. I dont know any of this for sure, but thats what I would think.
Mark