Quickest way to repair on site is make a new one, match the color, and do a quick install. It's nearly impossible to use any color to mask a stain that is darker than the base color surrounding it. Lighter, as in a white water mark, can be toned and faux painted if you have an artistic bent. But black on brown will be impossible until you get the black a lighter color than the surrounding wood. That means a two part bleach and possibly a complete refinish. Nothing possible on site. As I said, a remake is the easiest if we knew what that was.