What about someone who's Japanese? These questions are easy now, but in the context of Japan's occupation of Korea, perhaps they were more contentious.

Weird question since the belt system came from Japanese martial arts.

The Japanese martial art of Judo was the first martial art to use the belt system.

Karate is a martial art that came from China and it was practiced in Okinawa. It was later introduced to mainland Japan by Gichin Funakoshi. To make the martial art more Japanese, he adapted the uniform and belt system of Judo

Taekwondo is a pretty modern martial art that's heavily influenced by Karate; it adapted the uniform and belt system that was used in Karate

Judo is a Japanese martial art and it's popular in South Korea. It's one of the strongest countries in competitive Judo and there are a lot of black belt Korean judo practitioners.

So, of course a Japanese can get a black belt in Taekwondo.......