You can easily remove black stains from oak. You need gloves, rags, denatured alcohol, sandpaper, goggles, degreaser, bleach and a brush. Wipe down the wood with the alcohol and lightly sand the affected area. Then bleach the stain before drying the area thoroughly. This will remove most black stains./
Put on your gloves and goggles before getting to work. Use a soft rag dampened with denatured alcohol to wipe down the wood until it's clean.
Use the sandpaper to sand down the surface of the oak. This process should remove the finish. With the degreaser, remove oils or buildup that remains on the surface of the oak itself. You don't need to focus on removing the black stain yet.
Pour some of the chlorine bleach onto a second rag and apply it directly onto the black stain of the oak. Wait for about 10 minutes before wiping off the bleach. If you can still see the stain, repeat this process as many times as necessary until the stain is no longer visible.
Dampen a rag with water and run it across the surface of the oak. Clean the oak thoroughly and allow it to air dry normally.