Since you're wanting to make it black to match other accouterments on
your vehicle, consider that most, if not all, black-dyed anodization
will fade in the sunlight, and it doesn't take all that long. In
addition, depending on the aluminum alloy, the "black" may not be all
that black to begin with. Why not instead give the antenna a coat of
automotive enamel or lacquer? You can either mask off areas that need
to have electrical contact, or scrape them after painting. Perhaps you
can paint after the antenna is assembled. That way, you can get a
color that really matches other items.

Cheers,
Tom