There are several reasons why there may be a black residue in a Whirlpool dishwasher after a wash cycle. The problem can be taken care of without calling a plumber as long as the unit was installed correctly.
The black residue collecting inside a Whirlpool dishwasher is a result of bacteria growth caused by food particles blocking the drain or jets. When large particles of food are still stuck to dishes prior to the cleaning cycle, they may become stuck inside the drain opening or beneath the center spraying hub. Sometimes they even travel to the perimeter of the door and continue to decompose. Without clearing out the drain and cleaning the door and tub, bacteria will proliferate, turn black and stick to any available surface.
You can reach a rubber-gloved finger into the drain opening to feel for blockages. Unscrew the plate above the cycle programming unit and pull the dishwasher away from the wall to check the rest of the hose. With gloves still on, you can reach beneath the lip of the center hub and remove any large particles. To find out if the jets are clogged, you can insert a thin object such as a metal skewer into each hole, and clear out the debris.
All-purpose spray cleaner with bleach or a 50/50 mixture of white vinegar and water should be sprayed onto all the surfaces of the wash tub, door edging and seal. You can then run 1/2 cup of white vinegar through the longest wash cycle with the tub empty to clean and disinfect the water and drain lines.
The drain, door edging, jets and center hub should be checked regularly for debris, or any time there is a residue or odor in the tub. If your Whirlpool dishwasher model is old, your water pressure is low or you find that residue builds up often, you can try scraping and rinsing dishes carefully before running them through the wash cycle. A vinegar cycle should also be run once per month, regardless of the condition of the unit.