The black dragon society is like a club or a group of people who believed in things that were different from what other people believed. They started in Japan a long time ago, before you were even born. They were very secretive, which means they didn't want people to know they were part of the club. They had their own symbols and codes, like a secret language.
Some people thought the black dragon society was a good thing, but other people thought they were bad. Some people say they wanted to do things to help Japan, but other people say they wanted to control the government and make people do what they wanted. They wanted to have power and money.
The black dragon society is not around anymore, but some people still talk about it and think it was important in Japan's history.