OK the black stuff you get here is a fungus that grows on washing powder residue and the debris that is washed off your clothes. If it is not too bad you can smear bleach around the door seal leaving it on a couple of hours and then putting the washing machine into a rinse to wash it off. If however it is badly contaminated then the only option is to have the door seal replaced. The trick to avoiding this reoccurring is to once a month place a scoop of biological powder in the washing machine and without any clothes in the tub carry out a boil (90/95C) wash. The fungus cannot live above 70C so this destroys any spores or residue and prevents any further growth ~ this must be continued throughout the life of the appliance to keep it fungi free.
Would you now kindly rate my information to you using the smiley faces below, this allows me (once done) to close down the question on the system whilst at the same time letting you ask further questions on a free of charge basis.