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IF the 'black' is mold, then it will come off with a mix of 1 cup Bleach w/1 gallon Water. You can go stronger if you'd like. Then, rinse/wipe a good bit with water.
If this doesn't work, then the black ISN'T mold - It's dirt. From there, who knows what it is.
My best to ya and hope this helps.
Jay J -Moderator