Wash it many times. The black dye in karate belts can fade after washing and it will eventually turn to a shade of dark gray. Use a harsh laundry detergent that has no color protection or anti-fade ingredients and use the hottest water settings possible. Wash it over and over again until you get the desired results.
Scrub it. In a sink or large tub, scrub the belt using a brush with stiff bristles, such as a fingernail brush. You will have to scrub it and rinse it many times to see the desired results. Use a harsh laundry detergent and hot water. Heat helps to fade color.
Let it sit in the sun. Lay the belt out on an old towel or blanket in the sun on a very hot day. Turn it over regularly. You may have to do it many times, but eventually the sun will fade the rich color from the black belt.
Wait. After many years of hard practice and washings, the black belt will fade naturally. While it may be possible to fake a faded black belt, you will have more respect for yourself and your accomplishments if you wait for it to fade over time.