I have had black marks on my Mazdaspeed wheel for ages and I could not get them clean when I washed my car. Which really bugged me as the wheels are expensive. I decided to speak to the guy that changes my tires for me and he told me to use polishing compound and in about 10 mins all the wheels were spotless.
I though I would share even if some of you may find this obvious.

https://www.rx8club.com/attachment.p...1&d=1317400608
Here are the black marks that were all over the wheels
https://www.rx8club.com/attachment.p...1&d=1317400608
Just used a little bit of polishing compound which is about $5 and rubbed it on with a cloth.
https://www.rx8club.com/attachment.p...1&d=1317400608
The wheels all look like new now.

 

Thanks for showing.