Sand will slowly migrate to the bottom and the larger particles will come out on top. Lucky for you there is a literal "flourite black sand" product.
Interesting. I've been looking for a black sand that has the grit size of Tropical Eden's aragonite sand (reef-flakes or grand select). Would this Flourite black sand product be about the same grit size or smaller?
To get an idea, the aragasnow is the grit size of sugar.
(Picture taken from Google)