Russell, I've found that the blackest stuff scrubs off with a plastic brush under water, which leaves a grey (fairly hard) residue. This appears to give some level of rust protection. It may be a substance called "Pearlite". It can be polished off with a wire brush if you want the tool to look better, or it can be left on for a little added protection.
In using Citric, Acetic and Hydrochloric acids, I've always found the same residue present. I suspect that the black part is converted rust, and the grey underneath is converted steel.
HTH
Brett