The belt system used in karate was introduced some time during the late 19th and early 20th centuries, and most notably began among practitioners of judo, another Japanese martial art. A black belt signifies a high level of achievement, and the first black belt is granted to a karate student who has achieved his first graded rank, or "dan." A faded black belt often shows that a karate practitioner has worn his black belt for many years, and it has faded over time. But there is more than one way to fade a black belt, if you are interested in making a costume or merely seeing how it can be done.
Wash it many times. The black dye in karate belts can fade after washing and it will eventually turn to a shade of dark grey. Use a harsh washing powder that has no colour 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 washing powder and hot water. Heat helps to fade colour.
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 colour 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.
You can use a combination of these steps by washing the belt and then letting it sit in the sun to dry. Scrubbing or washing your belt many times may cause the ends to fray. This can cause some problems and make it more difficult to tie. If the belt frays too much, you may need to sew the ends to keep them from fraying further. If your belt is made from silk or a synthetic material, the belt may not fade as much as a cotton one, or may not fade at all.
Do not use bleach. Bleaching the karate black belt will cause it to turn a brownish colour, instead of looking faded.