Japanese started using the word 緑 "midori" to mean green after the war and increased contact with western countries. The word midori used to refer to freshly growing plants, like the English words greenery or flora. Otherwise both blue and green were 青 (ao) until pretty recently, with green considered a particular shade of blue. And weirdly even though the word midori used to literally refer to vegetation, the word ao (blue) is still used to refer to vegetables or plants as green.

This mirrors what happened in Chinese too, even with the same characters, although I'm not sure when Chinese started separating blue-green.