The color of a Karatekas belt indicates her rank and in theory, her ability. Each step forward represents a further accumulation of skills and knowledge. Outside of Japan each ‘minor’ Karate Level known as Kyu is indicated by a different color. Some Karate Styles also use black stripe attached across or along the belt. There is less emphasis on belt colors in Japan than elsewhere. There most Karate Styles use only 3 colors; White, Brown and Black.
A Karate Black Belt indicating a rank of first Dan and above wears a plain black cotton or silk covered belt, usually embroidered with the wearers name and/or school. In some Karate Styles a red belt is worn to denote a very advanced Dan rank. Belt colors generally get darker through each Karate level towards Black Belt. This may be symbolic of the time when a plain white sash (Obi) was used to tie a plain white tunic for practice. If a student trained often enough, the white Obi would eventually darken. This seems a likely reason for the Karate Black Belt to symbolize the most advanced Karate Level.