Hi. I'm Smitty. I am a professional appliance
repair consultant here to help you diagnose, and repair, your appliance.
This sounds more like a mold problem than a mechanical one.
There is a product called Affresh. It is primarily for front load washers, but it is designed to prevent mold growth in the units also.
Run a tablet of Affresh through the unit (crumble it up and place it in dispenser cup), then run a plain water only cycle, and then use some Dishwasher Magic in a complete cycle after that.
Between the two products you wont have any more problems with the black gunk.
Note: you may have to run two treatments to get rid of it all. Mold can hide in the darndest places. If you see any hint of it do this again.
Please let me know if you have any other immediate concerns.
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