You're right, though I would say that what we call green and blue were considered shades of the same color 青 (あお), rather than being "seen as the same color".
Japanese now has a word for green, 緑 (みどり), but according to a quick Google, that only really came into common usage after WWII. Interestingly, Japanese traffic lights still use blue to signify "go", and they are described as ao.
Yeah, you're much more likely to encounter this word as the kanji 青, though it is commonly stylized as あお on book/anime/manga/movie titles or shop signs etc.
青 is the noun, meaning the color blue (sometimes also 青色 ao iro), while 青い is the adjective form used when describing something as being blue.
“Blue—one of the four traditional colors originally established in the Japanese language along with red, black and white—historically encompassed items that other cultures would describe as green,” resulting in a kind of “grue” or “bleen.” Objects (e.g. apples) or persons (e.g. novices) that would be “green” in English are referred to in Japanese as being “blue” instead — and this includes traffic lights."