Glossary of Judo terminology
The Black belt is worn by Budo (martial arts which include Judo, Karate, Aikido, etc.) practitioners of the "Dan" class, with ranks of 1st Dan and higher.
Originally, it was customary to use Judo belts without ever washing them, and because the belts therefore became soiled (blackened) with repeated practice, it was easy to see who was a beginner and who was a veteran. Legend has it that this was the beginning of the "Black belt".
A "Brown belt" ranks below the "Black belt" because it has not yet become fully black (soiled). In the women's division, a Black belt has a white line around its center.