For some people's language usage, blue and green are adjacent colors, and thus defining a point that divides them is perfectly fine.
For other people, these are not adjacent -- for some people, there's a single color (aqua? turquoise?) between them, and green and turquoise are adjacent colors, as are turquoise and blue, and it's reasonable to ask about a dividing point between those adjacent pairs.
For those who don't use language this way -- do you consider red and blue adjacent, or do you consider purple (violet?) a necessary intermediate? Are you comfortable defining a point between red and blue, or are you instead comfortable defining a point between red and purple, and a point between purple and blue?
And for all I know, there are people for whom blue and green (or blue and red) have a distance greater than one, or greater than two...
I think there's another set of questions here -- why is "blue-green family" a thing in your mind, rather than "blue-yellow family"? Is there a "red-blue family"? "Orange-blue"?
Also, yellow-blue and red-green are opponents that can't be mixed because of how our retinas preprocess the signals from cones. Therefore, you obviously end up with blue-green (cyan), red-yellow (orange), yellow-green (lime) and blue-red (magenta, which actually doesn't exist on the light spectrum) families of related colors.
Russian speakers broadly consider sky blue / turquoise / cyan a distinct color right between green and blue
https://en.wikipedia.org/wiki/Blue%E2%80%93green_distinction...
Although the question "is the color distinct and basic or just a shade?" is very subjective. Is pink distinct or a shade of red/purple? Is purple distinct or a shade of red/blue? Is green distinct or a shade of blue? (it's well-known that in Japanese green separated from blue only relatively recently, with very bluish traffic lights and other quirks included)