Wash a spray bottle well to make sure there are no remnants of other cleaning products inside. Mix one part chlorine bleach with 10 parts water in the spray bottle.
Empty all laundry from the washer. Wipe excess water from the rubber seal with a rag.
Spray the bleach and water mixture generously onto the rubber. Wait 15 minutes.
Scrub away the black spots with a clean rag. Apply more bleach solution and scrub some more until all of the spots are gone.
Wash the seal with a clean rag and hot, soapy water to remove any residue. Run an empty cycle with just hot water to wash away any of the bleach lingering in the rubber. Dry the seal promptly.