Does Sand Blasting Stone come in black or dark gray? I've use number 1 and 3
size sand blasting stones , can get a 50lb bag for like $20. I had good luck with laying a layer of Latrite and capping it with a mix of both sands and grew everything ounce it aged .
A pretty common substrate is the coal slag blasting grit. usually something like 'black diamond', 'black beauty', 'black blast' or whatever. seems to work pretty well if you can find it at a supplier near you.
I don't know the grit sizes off hand, but I'm sure folks will drop in and let you know what works for them.