Believe it or not but this little trick really works.
First scrub the tires with a degreaser or detergent of your choice and a soft brush. wipe excess water and dirt off the rubber with a terry cloth towel. Let dry.
Apply a professional, premium grade, black shoe polish cream with a terry cloth towel. Let dry. Buff with a soft, horse hair brush, much in the same way you would shine shoes.
Apply a clear, shoe shine product afterward. The one I use is called the shine sponge—A sponge with shine product in it.
You can find these professional, premium products by doing an internet search for shoe polish or shoe care.
Your black rubber tires will be conditioned, look dark, rich black and super shiny!