Japan blue is the label that British chemist Robert Atkinson gave to the deep indigo shade of blue, which he perfected, used to dye cloth goods everywhere during his 1874-1881 stay in Japan. “Indigo is so widely used in Japan that one would be hard pressed to find a place in this country where people's clothes are not dyed blue."
Ai (zome): Indigo (dyed). The basic raw material is the leaf of the Polygonum Tinctorium. The Japanese process differs from that in other parts of the world in that the leaves are fermented to extract the indigo compound. The plant grows well in the western part of Japan (mainly Tokushima = old name, Awa.) Using sukumo (fermented Polygonum leaves = composted leaves) doesn't provide any shortcut to the rest of the dyeing process. Compared with other Indigo dyeing methods, the Japanese way requires more sensitive care throughout the process because they have to keep the bacteria alive through the entire dyeing session. The Japanese words some and zome mean dyed.
Samurai blue: Japanese national soccer team's team color