Black gravel would be awesome. I'm not sure about sand. I know a lot of people use it...I guess I just don't much about sand so am inclined to not trust it (like soil that peeps use in their planted tanks)
Sand is perfectly fine to use in most fish tanks...
BUT do not use it with Bettas!!! One of Rose's Bettas died because she swallowed a piece of tiny gravel. We all know how much they love to eat. So don't risk any substrate small enough for them to swallow.
Yes Sara was a totally white doubletail halfmoon female betta (even her eyes were white) and she at some of the black EcoComplete that is basically the size of coarse sand and I could see the black stuff inside of her right through her hide. It was sad but she died horribly. Nothing but pea sized gravel for my betta tanks now folks. The large stones are too hard to clean about.