EDIT: I apologize for the vague sourcing!
While the word 綠 to mean green has been attested as far back as 1000 BC, the idea that it was a separate color rather than describing a shade of 青 is relatively more recent. Wikipedia[0] indicates that it was adopted in the early 20th century in Chinese (as part of vernacular language reforms) and after WWII in Japanese, though these claims are currently marked with [citation needed]. While both are relatively recent, the usage in Chinese did have a longer period of time to take hold.
[0] https://en.wikipedia.org/wiki/Blue%E2%80%93green_distinction...
By the way there’s a variety of kanji that reads “aoi” in modern Japanese. 青い, 蒼い, 碧い are all “aoi”, but we use 青い mainly as blue today.
Have a source for that? This section[1] of the ja.wikipedia article could apparently use your revisions - both kanji are acceptable.
[1] https://ja.wikipedia.org/wiki/%E4%BA%A4%E9%80%9A%E4%BF%A1%E5...
If you want a source that the 1930 law used midori, see here, under「発祥期」:
https://ja.m.wikipedia.org/wiki/%E6%97%A5%E6%9C%AC%E3%81%AE%...
In 1930, not since the 1930s. The 1930 law used midori, but it was changed to ao in 1947 after that became the more popular usage. The page you linked was about the current law.
That adjective in Japanese means both blue and green. The Japanese language didn't have a widely-adopted word for just "green" - 緑 (みどり) "midori" - until after WWII.
Green apples or vegetables are still "aoi", for example. I lived in Japan for four years and never saw a blue traffic light.