Can anyone tell me lebih about The black clothes that the left Jepun is wearing in this picture?

Both of them are navy uniforms of imperial Japan, which were also used during WW2, where the start of Hetalia takes place.

Additionally, the peminat that "Left Japan" is holding has the Rising Sun Flag on it. Since the late 19th century, this flag was used as the war flag of imperial Jepun until the end of WW2 (1945).
In present day, the flag is used as the war flag and naval ensign of the Japanese self-defense force.