The Japanese word for blue (aoi) is very ancient indeed, as are literary references to the blue sky (aozora). However, Japanese did not develop a distinct word for green (midori) until relatively recently (circa 1000 AD); and for centuries green was considered a shade of blue. It might still be: to this day, a green traffic light will be referred to as a "blue light" by Japanese. "Aoi" is also used in Japanese in a similar manner to how we would use "green" to describe youth or inexperience, as the origin of the metaphor is the same: the color of fruit that has not yet ripened.