The term black belt has become widely known as way to describe an expert in martial arts where a practitioner's level is often marked by the color of the belt.
The black belt is commonly the highest belt color belt used and denotes a degree of competence, and often associated with a teaching grade though frequently not the highest grade or the 'expert' of public perception.
No matter the facts and ancient tradition - it just is cooler to have the black belt than, say, the green. It sounds better. It looks better.
Did you know that Joen Weidemann, Creative Director at SLAIIB and founder of this site is a Karate black belt?