青 encompassed both blue and green until the Meiji era, even now there are plenty of common words that use “ao” in place of “Midori” See: 青菜、青リンゴ、etc.

At one point the traffic law referred to 緑色 traffic lights, but was updated to 青色 post-war in the 1940s. It still means ao (green) not ao (blue).