[hnfong]

This discussion is weird, because the Kanji 青 in Japanese was borrowed from classical Chinese (presumably during the Tang dynasty around the 7th to 9th century).

So using Chinese as an example of "another language" having green-blue confusion isn't very meaningful since modern Chinese aka Mandarin also inherits pretty much the same 青 albeit popular usage differs somewhat.

Yet the fact that there's no blue traffic light in China just hinges on the fact that people don't commonly refer to the "green" in traffic lights as 青 in China (but you could). The word 青 is used to refer to a broad spectrum of green and blue in different contexts, just happens people generally refer to the traffic lights as "綠" instead. And since there's no ambiguity to "綠" there's no confusion of whether the traffic lights should be "green" or "blue", and thus you don't see blue traffic in China (hopefully).

[fenomas]

The word "ao" in Old Japanese already had the blue/green confusion before the character 青 was borrowed to write it, so if classical Chinese did as well I guess that would be two such languages.

[thaumasiotes]

> The word "ao" in Old Japanese already had the blue/green confusion before the character 青 was borrowed to write it

What would the evidence for this be? It's clear that the word "ao" does not derive from the Chinese pronunciation of 青. But its meaning could easily have shifted under the influence of the spelling. Do we have a source that writes the word without spelling it 青?

[fenomas]

This? https://ja.wikipedia.org/wiki/青#やまと言葉の「あお」