When you clean aluminium in the dishwasher, it oxidises. This oxidised aluminium is dark grey, rough and will come off on anything that touches it. Frankly it is pretty disgusting and you don't want it going anywhere near food.
While I am here, I am more than a little non-plused at Kitchen-aid for making attachments that contain aluminium rather than all stainless-steel. Replacements cost a small fortune so aren't really an option.
Here is how to clean all that nasty gunk off, and save your attachments from the bin.
You will need: Paper towels, cotton buds (optional) and Peek Polish - I'm sure there are other polishes that will work, I used Peek and so does the Queen, so you'll be in good company if you order some off of eBay forthwith.
Rub on the polish, just a tiny bit at a time and buff it off... that's a good word 'Buff'... Buff!
Use cotton buds to clean hard to reach places.
Make sure you rub well on every surface of the aluminium.
So after 30mins elbow grease and quite filthy fingers...
I am not the first person in the world to do this and I sure as heck won't be the last. I am really happy I found something that works.