#0000ff doesn't exist in nature. Human beings don't see color directly in hexadecimal values - indeed, additive color (RGB) as broadcast from a monitor and subtractive color as exists in nature are physically different processes. Rather, the perception of color is subjective and error prone (see a HN subthread on the color brown[0,1] or other examples of color illusions like the viral dress from a few years ago, or the red-grey illusion[2].
Blue is a social construct because what "blue" means is taught to us by the culture around us, and different cultures classify colors differently[3,4]. If one culture considers blue and green to be the same color, and another considers them distinct, that's not merely a disagreement over taxonomy, but concept.
