Challenge acccepted:

Blue light is defined as roughly 380nm-460nm range of wavelengths in electromagnetic radiation, however the human eye perceives blue when observing light with a dominant wavelength between approximately 450nm-495nm. Objects coloured blue typically reflect or diffuse this range of wavelengths more efficiently than others wavelengths, rendering these objects the appearance of the color blue.

Sources:

https://en.m.wikipedia.org/wiki/Blue

https://en.m.wikipedia.org/wiki/Visible_spectrum

I don't think that will help a person who was born blind know what blue means to people with sight.

I beg to differ, on a technical level electromagnetic radiation can be perceived as a physical force, similar to the sensations felt by an MRI machine. Your counter point appears to be around the emotions blue, or other colours, convey which is actually easier to explain than what a colour actually physically is. We use words to convey emotions all the time and to describe blue, adjectives like calm, cool, cold, sad, open, broad, easy, etc. etc. can be attributed to the colour blue.