I often soak walnuts and pecans in saltwater overnight before toasting them in the oven. When I do this in metal mixing bowls, I notice a black residue on the side of the bowls. What is this?
I can't say for certain, but my suspicion is the tannin from the walnuts is reacting with iron ions in the metal to create iron tannate. You can actually make a dark brown to black ink or dye using walnut hulls (higher in tannin than the nut itself) and rusted nails (or the like).