No, blue is what is collectively perceived as blue, while also not being collectively perceived as any other colour (or color if you are a “gray”). That’s how they came up with the objective, standard, scientific definition of blue above.
And the sky isn’t pure blue, it’s a quarter of the way between blue and green.
But there can’t be any cross-cultural confusion, because it is written in English. Vietnamese or Japanese people either know what the English words “blue” and “green” mean, or they don’t speak English at all and wouldn’t be reading this story.
And if the story was written in Vietnamese, it would use “xanh lá cây” which means green and “xanh dương” which means blue, rather than just “xanh”. Just because people normally use the same word to describe two different colours, doesn’t mean they can’t see the difference between those colours, and don’t have ways of describing the difference when they need to.