The deep blue color of aizome (indigo dyeing), is often referred to as the color of Japan. Made from the ai (indigo) plant, a type of tade (smartweed) grown in Japan, aizome has also gained a great deal of popularity worldwide. Although indigo comes in an array of hues, the most popular is one that is commonly called Japan blue or Hiroshige blue in Western countries. The name originated from the unique indigo blue of the sky and sea that the Edo Period artist Utagawa Hiroshige frequently used in his woodblock prints.

Natural indigo dye can be traced back to ancient Egypt. However, the indigo used in Japan was originally said to have been exported from China. The golden age of indigo was during the Edo Period, when people used it in everyday life, from tenugui to working clothes. Indigo-dyed items were considered to be very sturdy, and were even said to be less likely to get eaten by months.

The most famous of the indigo dyes is Awa-ai from Tokushima, where most of the indigo plants are grown. However, even though plants no longer grow in the Kanto area as they once did, Bushu (today's Saitama) aizome is still made using original methods of dyeing. Unfortunately, since the Meiji Period, chemical dyes, which do not have the subtle gradations and texture of natural dyes, have become popular and have replaced some of them.