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Ok not good news once its gone black you wont get rid of it. What it is is bacteria. It builds up on the door seal normaly when its wet and the door is closed as there is no air in there so it grows like that. But once its there the black marks wont go.
You can try a few service washes which you do on 90 degs with no clothes in and a bit of powder. This will kill the bacteria but the marks will stay there.
The only way to get rid is to replace the door seal. As you havent posted the model number i dont no the exact price but bosch seals are about £30 then you will have the labour if getting someone in to repair which could be about £40.
But once you have a new seal on when its finshed dry the seal out and keep the door open a bit to allow the air in and it shouldnt happen again.
If you need anymore info just get back to me.