Just to expand the explanation, blue and green were both just “ao” in Japanese until relatively recently (relatively still being a fairly long time), when “midori” started becoming more popular to refer to green.
As for why the traffic light is still called “blue” despite being green, one need only ask why there is a very popular rhyme scheme in English that goes “roses are red, violets are blue…” when violets are violet.
Yeah, you’ve overshot and ended up in Japan.
Edit: Explanation: the Japanese word for “green light”, “aoshingou”, means “blue light” even though their lights are green too.
Just to expand the explanation, blue and green were both just “ao” in Japanese until relatively recently (relatively still being a fairly long time), when “midori” started becoming more popular to refer to green.
As for why the traffic light is still called “blue” despite being green, one need only ask why there is a very popular rhyme scheme in English that goes “roses are red, violets are blue…” when violets are violet.