Discolouration
You will probably notice this on the plastic components in your dishwasher, the spray arms or cutlery basket. If you try to rub this discolouration you will probably notice that it will smear.
This effect has been caused by washing articles made or silver, particularly cutlery. Minute amounts of silver can dissolve and this oxides and forms the sooty deposits on the plastic parts of your dishwasher. Read more about this here…