foundry sands, would be more expensive than play sand or pool filter sand, but ... you could get sand that is round to sub-round (which is good for your fish) ... i'd also stay away from anything "slag" Spherical Ceramic Sand for Foundry - Opta Minerals CERABEADS®, a cost effective replacement for zircon sand.
not black, but i would be pretty sure it's density will garantee it stays ontop, ... maybe other companies offer simular products in darker, or even black colors
i don't know the densities of any foundry sands by heart
i have considered ceramic beeds
coal slag, being 'angular' isn't that going to be rough on any fish or critters that deal with the bottom of the tank ?
i've played with the idea of glass bead sandblasting media - but it's clear, ... it should take on the color of whatever is underneath it
for black, i would recommend chromite, if only as it's not as abrasive
i would keep looking for alternatives if your concerned about cost, i'm sure foundry sands & sand-basting sands to use for substrate caps are more on the expensive side, ... oddly still cheaper then what you get from the LFS
i have no idea about densities, for anything other than the cerabeads (ceramic beads, rather fine sized from that company) are expected (personal guess) to remain above whatever else you use for a substrate.
that only solves where things are as you move & rearrange your tank on an (ir)regular basis (your choice, not what i would do), it does nothing for color, where i would again recommend looking for other companies that may have it in darker colors
i like alibaba.com for it's options, a search for low density foundry sand, "yay" they have black, ... $350-400 per ton, ... i don't think anyone here needs a ton of the stuff, and if everyone pitched in, i'm sure there would still be surplus.