Does anyone know what this is and what the best way to remove it is? I've been using aircraft remover to strip the clear coat off of my wheels, and this black stuff is not coming off. Not sure if it's some kind of road tar or something else, but nothing I have used has gotten it off. Any tips?

 

Yup. That's just good old fashion aluminum corrosion. Since you got the clear coat stripped off, you can use alumi-prep to remove the corrosion. You'll need to scrub it a few times to get all the black stuff out of there. And then polish it and re-clear coat it if you want.