Many (normal) people are thinking that a Black Belt means you are a Master. This is not true. Originally the Black Belt meant that you have the basics down and you can start the real Training now.
You receive your black belt when you reached the rank of first Dan, Shodan. This means roughly translated first step or beginner rank in japanese. Before that you would have gone through the kyu levels. Which can be translated as “classes”. Atleast in most western Dojo. In Japan many Dojo only bother to test Kids for Kyu levels, to keep them motivated. That’s also the original reason for the multiple belts in different colors.
This System, the dan-i System, got popularized by Kano Jigoro the founder of Judo. He himself adapted this type of System from the board games of Go and Shogi. There this System was used to determine how much of a handicap the better Player would have to overcome against a lower ranked player to keep it exiting and a challenge for both.
Kano didn’t implement this System because of shits and giggles of course. He had a quite solid reason. In most martial Arts dojo before the dawn of Judo the teacher would have known every single Student of his personally. He would know their skill level, strengths and weaknesses. There was no need for Rankings as the dojo and Ryuha were so small in Students that everybody very well knew everybody else quite well or atleast has heard of them when they were from another Branch.
But that changed dramaticly with the dawn of Judo. Judo with a lot of backing from political power behind it was implemented in most Schools and Universitys to install the values of Bushido into the japanese people from young age. Dojo, Study Groups, School and University Clubs popped up fast everywhere in Japan. Kano got so many new Students in such a short time with different expierence Levels that a new System was needed to evaluate them and to ensure a safe training enviroment.
He needed to discriminate between people who had the basics down and could already Fall safely after being thrown, these were the first Dan Rankers. And the people who could not and had to master simple basic techniques, the kyu grades. This was easily marked through a white Belt for Beginners and a Black Belt for people who could train safely without a lot of supervision. And over time this ranking System became more sophisticated. As similar as in the original Board Games your Dan Rank would mirror your fighting strength you would have proven in Randori practice and official shiai(matches). When a Shodan would throw enough people regulary that were ranked second Dan he soon would be promoted for example.
When you would open a Dojo in Japan, only holding the Rank of Shodan you wouldn’t attract many Students. More likely people would just laugh at you. Of course not in your face, i mean come on it is Japan. But behind your Back. In Japan roughly with a Sandan you would be considered as a kind of “Junior” Teacher. Most people would only dare to open their own Dojo with atleast the Rank of 5th Dan. And to be considered a real Master(Shihan or Hanshi) of your Martial Art you would need atleast a Rank beginning with 6th Dan and the approval of your organisation.
So… how did we end up in the West with this wrong understanding of the meaning of the Black Belt?
The american Soldiers after WW2 came back from Japan with some Training in Karate, Judo or any other Martial Art. As they were Training regulary they got promoted quite quickly and got their Black Belt.
Then, after coming back they would open their own Dojo with themselves as Masters. Far away from their original Teachers they with only the first or second Dan would have to be the Master. And with this the picture of the Blackbelt as sign of mastery was forged in the western mind and propagated by Movies and other form of popular media.
When you started your Training at this time in the West, Black Belts were rare. And those people with one in the Dojo would obviously teach. But in Japan this situation is quite different. There where even the modern Gendai Budo have a far longer History it is quite normal for people to see many Blackbelts, 3rd, 4th and even 5th and 6th Dans Training in their Dojo. The Senpai in those Dojo are not your typical Brown Belt, no they are probably 4th Dan. And the Black Belt for itself is nothing special.
Also while in the West most modern and “proper” Organisations will only give you a Black Belt after around 10 years of Training when you have shown enough maturity.
In Japan it is quite common to get the Shodan/first grade Black Belt already after 2 to 4 Years of regular Training. As it isn’t seen as a rank of Mastery, there is no big need to prove your maturity. Even kids can get a Shodan in Japan. And no, they don’t train in a McDojo.
So what is the truth behind the picture of the Blackbelt as the Rank of Mastery? Well this is just my opinion of course. But in my view it was simply because at the Beginning there were no real Masters of these Arts in the West. Of course with the Exception of Teachers who were send from Japan to the west especially with the reason to teach.
But these were rare. The first western Pioneers basicly were just a bunch of Amateurs who get the basics down, atleast when we view it in the big picture. This should not degrade their important work of course. And many followed the path further and developed into real Masters over time. But some others were not, and those were the ones who would put the black belt by itself as the sign of mastery and promote this misconception to blow up their own ego.