However careful you are with your beautiful hardwood floors, you're likely to get a water stain on them at some point. Surface water stains are usually white in appearance and can be removed easily with a soft cloth and a mild abrasive. But water spots that appear dark in colour have penetrated the finish and require more elbow grease and chemicals to remove.
Mask the area around the black stain with masking tape to protect the floor that is not stained. Also surround the stain with an old towel to prevent damage to the unstained area of the floor.
Put on protective rubber gloves. Rub the area with a fine steel wool pad and white spirit, both of which are available at a hardware or paint shop. Use only enough white spirit to lightly cover the stain. The combination of rubbing with the steel wool and the white spirit should take the finish off the water spot.
Wet a rag with vinegar and apply to the stain. Allow the vinegar to sit on the black stain for five minutes. If the vinegar seems to be lightening the black water spot, apply more vinegar and let it sit for another five minutes. Continue applying vinegar and waiting five minutes, if it looks like the vinegar is working on the black water spot.
Mix a 50/50 solution of household bleach and water and apply enough to cover the stain, if the vinegar doesn't work on the black spot. Wait five minutes to see if the bleach works to remove the stain.
Apply oxalic acid to the centre of the spot. Oxalic acid is a strong bleaching agent available at a hardware shop. If it doesn't bleach the stain the first time, apply several more times until the spot is gone.
Neutralise the oxalic acid by applying vinegar once the stain has disappeared. Wash off the vinegar with a small amount of water. Wipe up any excess liquid with a rag. Allow the spot to dry.
Remove the tape and towel. Sand the spot with sandpaper, then stain it with wood stain that matches the floor colour. Apply stain along the wood's grain with a rag, and wipe off excess with a second rag. Use as many coats as necessary to reach the desired colour.
Let the stained wood dry overnight. Do not walk on the stained floor.
Apply wood finish with a bristle brush. Apply three thin coats of finish, allowing each to dry between coats.