I have same spot from severial years ago.It's not mold but a stain that should be removed using Bondex Wood Bleach with Oxalic Acid. A friend of mine removed the black from his front door oak wood. I haven't tried it yet but plan on removing the poly finish, applying the bleach than re-staining and sealing with a poly finish. The stain is "basic" or the oppisite of acid. The acid will neuterlize the stain and remove.
If you try it before I get to mine, please advise your results.