Hi SIXSKB

Just on the off chance that it is not black oil staining there is a good chance that it may be Iron/Water staining and one of the first things I try is Oxalic Acid or a commercial Wood Bleach. Though I do have to say that removing stains from some woods the outcome is usually in the lap of the gods if it works completely but you can usually lessen the visual impact.

The only lessons in my life I truly did learn from where the ones I paid for!