Why are my wheels black - it's not brake dust

I have slowly watched my wheels morph from stock silver to a flat black. I'm not talking subtle brake dust black. It looks like someone has been sneaking up to my house at night and slowly spray painting them one little bit at a time.

I have pressure washer them. I have tried Adam's Wheel and Tire Cleaner, but it just makes them look freshly painted(black).

Does anyone know what causes this, and how do I clean it?

 

I figured I'd add a photo of what the suds look like when applying Adam's. This wheel is a different one than in the previous post.
With Adam's


After rinsing it off with a pressure washer

 

Sorry to spam my own post, but each wheel is revealing new info.
I just sprayed the last one and the silver is coming off before my eyes.

 

Edit. Typing while you posted more.

I would say that the silver wheel paint is what’s oxidized and flaked off until it’s very nearly gone, leaving only primer or a previous finish coat. Any idea if the wheels have been refinished before?

 

To be honest I'm not sure. We have owned it for 12-13 years, and the family we bought it from didn't seem to concerned with an aftermarket look. Meaning it was completely stock.

 

Thanks for all the quick responses!

I'm thinking it's primer as well. I don't remember sequoias or tundras coming out with black wheels from the factory in the early 2000's.

Kind of funny, because I have another set of 4 wheels identical to this I have been preparing to paint for my 1st gen tundra. Looks like those will be practice for the blackening sequoia wheels.