[second_brekkie]

Maybe Korea also had blue traffic lights in the past. But they still call green traffic lights 'blue' even though all of them use a green coloured light.

I think the use of blue to mean both blue and green has also been present in Korea for a long time.

I wonder where it came from? In my limited experience if Korea and Japan share something culturally it often has its common derivation in Chineese tradition.

[iopq]

Both come from the Chinese term 青 which in modern Chinese would by itself refer to colors anywhere between sky blue and grassy green

青天 - clear skies

青草 - green grass

清葡萄 - 청포도 - white grape

[thaumasiotes]

As vague as 青 might be, I'd have to allow that it's a more accurate description of the color of a grape than "white" is.