The colour of human hair is determined by two varieties of melanin, the pigment which also dictates our skin colour. And the bad news for those wanting naturally blue hair is that both varieties come in only boring colours.
The most common is eumelanin, which is brown-black; slightly more exciting is pheomelanin which is reddish-yellow, and is present in high levels in the hair of redheads and blondes. As to why we’ve ended up with just these two varieties of pigment, no-one is entirely sure; the best guess is that they evolved to improve our ability to blend in with our surroundings, vital for avoiding predators in the African savannah millions of years ago. Anyone wanting their hair to transcend the demands of evolution therefore has no choice but to go to the shop and buy more exciting hair pigments at the chemists.