Your picture did not load so I couldn't see what you have, but are you sure it's mold and not mildew? To many people they are both similar looking. If it's black mold it should have an odor. Make sure you use a good respirator when cleaning it (not just a common dust mask). Bleach works good on the mold but I don't know how it will effect the wood. It might not hurt it but you should always do research ahead of time to be sure.