Blue hair (青髪) covers the whole gamut of subject matters described by it; namely the presence of the substance commonly known as hair, of the blue variety.
'Blue' is an English word derived from the old Germanic word 'blao,' and is an example of a curious mental phenomenon known as 'colour,' in British English, and 'color,' in US English. These are English words derived from the Latin word, 'color.' Colour refers to the brain's interpretation of light making contact with the eye, with the cone (an English word derived from the Greek 'kônos') cells of the retina interpreting its position on the spectrum based on wavelength, while the rod (an English word) cells work on a separate axis running from black to white. The term 'blue' is broadly understood to apply between the wavelengths of 450 to 495 nanometres.
'Hair' has a whole article written about it already, on Pixiv Encyclopedia. To better understand what 'hair' means, it is suggested to read the article titled, 'hair,' on Pixiv Encyclopedia. Though it is worth noting that said article, on Pixiv Encyclopedia, and the tag it is written about, on Pixiv (not Pixiv Encyclopedia), are generally understood to refer to specifically human, or humanoid hair, meaning the hair that is found specifically on humans, or humanoids. This article, on Pixiv Encyclopedia, is much the same way, referring to blue human, or humanoid hair. To refer to hair that is not human, or humanoid, the term 'fur' would be more accurate. 'Fur' has its own, shorter article, on Pixiv Encyclopedia.
Blue hair, therefore, refers to hair (which can be read about on Pixiv Encyclopedia) that reflects light at a wavelength of somewhere between 450 and 495 nanometres. Hope this helps.
