I would not go with black sand here. It will impact the Corydoras panda. I had this species in a 70g tank many years ago with a black substrate, but it was not until I moved them (for other reasons) into a tank with a natural sand (buff/tan) that I realized what a difference the black made. They attempted to blend in, and this means they produce melanin to darken, and long-term this is stressful. The fish is continually working to darken so it will not be so obvious. White is also bad, for the opposite reason.
A natural sand that is inert and soft. There are aquarium sands like this. I have used Quikrete Play Sand for years, and there have not been any issues I could trace back to the sand. It comes in a natural buff shade, and also a dark grey. I happened to have the latter and it did not produce the problematic reaction that the black did. Available from Home Depot and Lowe's, a 25kg (50 lb) bag for 5 or 6 dollars.
Ian Fuller on CorydorasWorld has often written against black sand. No habitat in SA has black sand.
