Nothing really you can do. My old Roadking and my last Glide were Black. It is a pain in the *ss to keep clean, plus every little scratch shows. I used to worry about it like others said, but there are no easy solutions other than ride it and enjoy it.
Or clean it, polish it and keep it in the garage away from dirt......
I had a friend actually tell me he didn't want to ride cause the roads were too dirty after a rain! Just ride it, if it bothers ya hose it off when you get back!! Once you go black you never go back!