Repairing Dark Wood Stains

I have successfully used three chemical bleaching methods. Go to your local real hardware store for the supplies. I suggest you protect yourself with rubber gloves and eye protection.

Bleach contact will dry your skin and feel like a burn, so rinse ASAP if it gets on you. Start with weak dillutions and work through stronger concentrations until you achieve the desired amount of removal. You must let the area dry to judge the result. Be careful how you apply the solution as it will remove the natural color of the wood as well.

I use a sponge with a abrasive pad on one side, a wash cloth, or steel wool and carefully rub it into the dark area. For small spots padding on with a folded up paper towel or a cotton swab is more precise and deliberate to avoid bleaching unaffected wood as much as possible. The grain will raise so sanding may be necessary after the area is completely dry, depending on your needs. More than likely staining will be necessary to blend the area back to match the surroundings. I use 1 part bleach with 9 parts water (10%) stepping up to most concentrated solution, 100% before moving up to the next regime.

Weakest: household bleach
Strong: oxalic acid (granular powder before mixing in water)
Powerful: 2 part bleach. Part A and B. (Note: this gets hot when applied.)

Oxalic acid is usually enough. It will get black stains from water out of an oak floor!