Aristotle writes about this in Metaphysics (I think...college was a long time ago). There is an absolute, pure version of everything...the platonic ideal. Our souls or whatever recognize things by their relation to the platonic ideal. There is a TRUE blue, and it's our understanding of this that allows us to discern shades.

Scientifically speaking, the center frequency of the blue section of the spectrum of visible light is probably "Blue"