I get calls like this at least twice a week. With newer washing machines, this is becoming a big problem.
Let’s start by looking at the reasons for it.
1. Over the past 20 or so years, a lot has changed in the way we wash our clothes. We now frequently lower the temperature of the wash cycles to 30 degrees or less, ensuring that neither the door seal nor the washing machine ever receives any warm water.
2. Because modern washing machines use so little water, the rubber door seal is never thoroughly cleaned.
3. Nowadays, laundry detergent rarely contains bleach, which kills bacteria, mold, and germs. Since we no longer use very hot water, there is no longer anything to kill bacteria and germs.
The issue is brought on by this. The bacteria and spores that are growing in your washing machine’s warm, moist, and dark conditions are what are causing those horrible black marks and stains on the door rubber.
What steps can you take to avoid the issue?
Try washing once a week at 60 degrees. To help eradicate dust mites, the Good Housekeeping guide suggests washing all bedding at a minimum temperature of 60 degrees.
Mold and bacteria in your machine will also be eradicated by the hot water.
Get the door rubber dry. 
To keep your machine’s door seal clean and dry, wrap a dry cloth around it once or twice a week. To allow fresh air to circulate around the drum and seal the door, leave the door slightly open.
How to get rid of the black marks and stains?
The only method I know of to remove the black spores and bacteria from the machine door seal is regular household bleach. However, the mold, marks, and stains must remain in contact with the bleach for at least eight hours with rubber door sealer.
I soak a cloth in bleach, lay it on top of the door seal stains, and leave it there overnight. For great results, this may need to be carried out two or three times.
Regardless of what you decide to do, bleach can be dangerous, so use it with caution. You will damage the door seal and be out of pocket if you try to scrub it with anything abrasive.
