Cleaning up aluminum wheels?

I have the Centerline Convo Pros on my car. For some reason when I got the rims they had black stuff in the grooves. Its kinda hard to explain but if you look at the link below you will see what I mean. They have the rivets that go all the way around and outside of the rivets is kinda lumpy. Between the lumps is black polish looking stuff that wont wash off. What can I do to remove this?

http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=3613&prmenbr=361