Possibly nothing to do with this, but here goes! When the sand from certain foundries is delivered it is the same colour as normal sand, ie; Sandy! After it has been used for making moulds and had molten steel (or cast iron) tapped into it, the sand gets ‘burnt’ or turns black! I know for a fact that the black sand gets sold on, as it’s no use for a foundry anymore.
The last place I worked at, we had to go to great pains to keep the black sand apart from the new, sandy coloured sand!! 