Of all the options in coloured contact lenses, blue is the most popular. Strange, then, that blue eyes are becoming less common1. Perhaps the increasing scarcity is making them more desirable.
Anyway, blue eyes aren't really blue. There's nothing inside the eye that you can say is 'blue'. The reason they appear blue is .. well, it's a trick of the light. Actually it's the same trick that makes the sky appear blue (and if you go up you won't find any blue bits there either). Seen from space, the sky is black and the sun is white. No blue. No yellow.
(While we're talking about space: moonlight is blue. It's just we don't see the colour2 because at low light we mainly use our retinal rods, and they see in black and white.)
OK so what makes brown eyes? That is a pigment: melanin, the same pigment that makes us tan or makes naturally dark skin. Brown eyes contain melanin and that is perceived as the dominant colour. Blue eyes, then, are eyes that are missing melanin.
So what's the downside of blue eyes?
Blue eyes may be popular, but there's a small problem. Dark colours absorb light, light colours reflect it. Light is energy and energy does all sorts of damage, so the brown around the entrance to the eye absorbs some of the energy and protects those with brown eyes. Blue eyes are less protected, more sensitive. So blue eyed people tend to screw up their eyes more in bright light. Do that often enough, and you'll get crows feet earlier (see crows feet article).
If you move a lot between natural outside sunlight and the interior of buildings, this can be a particular problem. Outside light is about 10 times brighter than indoor light, so a blue eyed person will feel that contrast much more keenly as they walk outside. Consider wearing photochromic lenses that react and assist the eye in coping with glare, especially if you are prone to migraines where light is a trigger.
It's not just light, though. Blue eyes are more sensitive to cold winds and smoky rooms too. Simply put, blue eyed people feel things more.
There's also a more serious problem. A lighter iris colour comes with a higher risk of age-related macular degeneration (ARMD) & progression (so check our article on what you can do to help prevent ARMD) and uveal melanoma.
On the upside, blue eyes are associated with a lower risk of developing cataracts.
Originally, we all had brown eyes. Then one person popped up with blue eyes. Basically all blue eyed people are descended from one person who lived in the northwestern part of the Black Sea region 6,000–10,000 years ago.
(If you really want to know why blue eyes are blue, the melanin of brown eyes exists in a frontal layer and also at the rear of the stroma (the coloured, filamented part of the eye). In blue eyed people, there's less melanin in the front layer, so light passes in to the rear layer of cells, the longer wavelengths of light are absorbed more than the short wavelengths, so the light that is reflected back is bluer. Then that light is subjected to Rayleigh Scattering where it interacts with microscopic particles to create a blue colour. Rayleigh Scattering also gives us blue skies. Check out the Tyndall Effect too: the blue of motorcycle exhaust fumes is the same sort of thing (but it's a little less romantic.))
1 Among American white people, blue eyes are becoming notably less common (approximately 57% of those born around 1900 had blue eyes, 34% born around 1940 had, and now 22% have).
2 Cameras do though