"Scientists believe that every person with blue eyes shares a common ancestor. This eye color first originated somewhere between 6-10 thousand years ago as a result of a genetic mutation. The mutated gene interferes with the body's ability to produce melanin in the iris of the eye. Melanin is a pigment that provides color to the eye, skin and hair. Since less melanin is produced, eye color manifests as a blue. This mutation affects only the iris of the eye and does not stop melanin production, only reduces it.
Interestingly there are no blue pigments in the eye. Blue eyes, as well as green and hazel eyes, appear a certain color because of a principle known as Rayleigh Scattering, which is the reason the sky appears blue, even though it isn't actually that color. Lighter eye colors, like blue eyes, are largely affected by the colors and light around them. The appearance of the eyes can vary from moment to moment."