Black discolouration is mainly seen on plastic components of the dishwasher, mainly the spray arms and cutlery basket. The plastic will look as if it is covered in soot and rubbing this will cause a smearing effect.
This is not a fault with your dishwasher and the cure for this problem can be found in the notes below.
Usually the result of:
Silver oxide deposits from dissolved silver in the wash water as shown above.
Washing silver cutlery
Cause & Prevention
When silver cutlery is washed in a dishwasher minute amounts of teh silver can dissolve, especially where detergents containing oxygen bleach are used. The dissolved silver is oxidised and deposits on the plastics as black silver oxide which, after a significant number of cycles, turns the plastic black.
Leaving wet dishwasher fro a long period with the door closed will cause mildew just as it would in a fridge.
There is no particular treatment that will remove the silver deposits although cleaning with household cleaner (such as Flash etc.) may improve the appearance.
Mildew can be removed by repeated use of high temperature cycles. Rubbing the mildew with hot soapy water will speed up the process.
The only way to avoid silver deposits becoming a problem in your dishwasher is not to wash silver in it.
Mildew can be avoided by leaving the dishwasher door slightly ajar when not in use, allowing the machine to dry out.