If you polish aluminum up it takes quite some time to go "black" - a very long time! Look at all the pipes and mufflers and headers out there from places like Mac's and such, they are not painted or anodised, just plain aluminum and they last years before they need a little polishing up. Also most if not all aluminum spinners are the same way.

I have tried high temp BBQ paint but even that goes soft on me, so I just leave em' bare.

AJC