Hi SWAMPMAN
It sounds like the black stains are the results of rusting Iron/Steel and the tannin from the wood. The usual way of removing such staining is Oxalic Acid applied to the stained area with cotton buds, there is another method if the staining is not to dark fresh Lemon Juice can also work though if it is large very dark patches go to the Oxalic Acid first.

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