They don’t. No one does. Blood does vary in color—but only from bright red to dark red. The fact that some blood vessels appear blue beneath the skin is actually trick of the light. Much in the same way that the ocean looks blue even though water itself has no color, when light passes through the outer layers of our skin and bounces off a blood vessel, the frequency most likely to bounce back out to our eyes is blue. That does not have any effect on the color of the blood itself. However, this misconception has long been a tool for supporting class distinction. Starting as far back as medieval Spain, being able to see veins of “sangre azul” beneath pale skin was a mark of a privileged, sheltered life that was unavailable to the sun-tanned working class. And though now we live in an age where sun-starved office workers dream of luxuriating out on the beach somewhere, our blue-blooded illusion of the upper class remains.