Liquid laundry bleach will remove most stains from wood-wood-finishing stains as well as ink, water, and other marks. Strip old finish from the stained area or the entire surface.
When bleaching a dark spot, it’s best to treat the whole surface so that you don’t leave a light area surrounded by dark. Apply the bleach full strength to the raw wood. Leave the surface wet for 20 minutes, then neutralize it with a solution of half vinegar and half water.
If after 4 hours a stain remains, re-bleach and re-neutralize the dark area. Before sanding the surface and applying a new finish, let the wood dry 24 hours.
To lighten naturally dark wood, use a two-part commercial wood bleach. Because such preparations contain hydrogen peroxide and lye, they are very caustic. When working with one, follow the directions on the package carefully, wear protective gloves, and work in a well-ventilated place.