Hello; Around 40 years ago a gravel substrate was available comprised of a crushed granite. It was made of black/white grains. I am still using some of it today. It is a bit too light to really highlight some fish colors but much better than most sands. My best estimate is that it ranges from about 3 mm to 6mm (perhaps from bb size to small pea size). Problem being that I have not seen it for sale in a long while and may not be available.
Around the same time I bought some pure black gravel which has been my favorite for decades. I made two mistakes. One being I mixed some in with the above mentioned gravel in order to have enough (about 4 inches deep) to do a planted 125 gallon. The other being I did not buy several more 50 pound bags as it also can not be found any longer. I have enough to do a deep bed in a 20 long. (Note to young fish keepers: The hobby moves on and sometimes abandons the good stuff. That I now wish I had a few more hundred pounds of that pure black is an example. Another being that I can no longer find the heaters that mount on the rim of a tank, only submersible type now. Also not many, maybe none, of the air operated HOB filters. I don't like it, but I guess things happen that way. [JC]
Sand can work and be cheap. I use to get construction sand. Got 3/4 ton five years ago for $15 USD for example to bed some pipes. I use to make a sieve of screen material and dry sift the construction sand. Dry sifting is almost critical in order to get rid of the fine powder and very small stuff. Adjust the size of the screen to get the particle size wanted. With any luck there will be enough dark to make the fish colors stand out.
I have not used this but have seen posts about a black substrate on this forum. I think it is called "black diamond." Some sort of industrial byproduct sold as surface blasting media. Some have reported using it. Do a search.
good luck