hi Tina, we have had the same problem with these black marks , which show up more on the whites!! Hotpoint say it is detergent sludge due to the water in our area being soft! Thet first recommended running a hot cycle without any garments in the washer, which we did, but in the short term did not help. They also persueded us to purchase some detergent/limescale cleaner from them at £10 for 6mths supply, and i must say that after a couple of weeks using these sachets every couple of days, the problem has cleared.-hope this helps.
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Mark Kennedy, May 2007