To make your rubber floor mats black again there are specific floor mat cleaners or black trim cleaners you can purchase. Or you can use household cleaning supplies.
Remove the mat from the interior of your car before you start cleaning. Wet the mat with a hose to clear off debris. Next, you can spray the mat down with your purchased cleaning supply, or use a homemade cleaning solution of dish soap and water.
Scrub the cleaning solution with a brush until happy with results, then dry the mat with a towel (or let airdry).