Black teeth symbolized the Japanese aristocracy
Good dental health has been and continues to be a symbol of hygiene, beauty, and health across most cultures. They say smiles can win hearts. 20th century Japanese probably realized how important dental health is to express happiness, health, and status. So they would do the most rational thing to preserve their pearly whites as soon as they hit puberty, which was blackening them!
The trend was collectively supported and accepted by Japanese aristocrats. For almost 1,500 years, black teeth were the thing among Japanese higher-ups. Quite contrary to the western ideology of beautiful teeth, the Japanese would often charr their teeth pitch black. And all this time, we made fun of the Brits for their dental hygiene and habits!