I have inherited my mother's big Kenwood Chef mixer - about 30 years old and still going strong. Only used it once or twice and washed the K Beater attachment
in the sink. However, last time I used it I washed it in dishwasher. When I got it out to use it for last week's big bake-in I picked up the K beater and found my hands covered with a black deposit. I put it in the dish washer which didn't help. More black deposits on hands. Resorted to Google. Apparently, it was the dishwasher and its detergent that had caused it. Tried scrubbing it with ordinary washing up liquid then Brillo pads. Nothing worked. Thought I'd have to buy a new one. However, back to Google.
Recommendation: soak item in 1/3 white vinegar + 2/3 hot water for 10 minutes. Ended up soaking overnight as the situation was bad and then a good (non-dishwasher) wash in hot soapy water. It's done the trick. Nice clean beater. No black deposits.
So if you ever have this problem with your mixer or any other aluminium items that you have inadvertently dish washer-ed this would seem to be the cure.
I'd noticed recently that a lot of aluminium pans have a recommendation to wash by hand. Now I know why.