Fallacious yet widespread and documented beliefs courtesy of Wikipedia.
The black belt in martial arts does not necessarily indicate expert level or mastery. As introduced for judo in the 1880s, it indicates competency of all of the basic techniques of the sport. The first five ranks all have black belts; holders of the third rank can act as local instructors and may be addressed as sensei. Holders of higher ranks in judo and other Asian martial arts are awarded belts with alternating red and white panels (6th to 8th dan), and the very highest ranks with solid red belts (9th and 10th dan).