You guys have certainly proved that black blasting grit bought today, where you bought yours, works fine with corys. It is possible that the grit I bought was a different brand, and it certainly was several years ago. I was working then as a DIY carpenter on my house, so my hands were pretty tough then too, and I sure had lots of cuts, both from cleaning it and from planting it. I stopped using my hands to stir it as I cleaned it, because it was getting too painful.
This may be like using kitty litter in a tank. It can work great or be a disaster, depending on the specific kitty litter you use.
I may some day use black blasting grit again, now that I know it can be workable with corys.