Jennifer Fisher Wilson in The Smart Set:
But just what blushing really reveals, even the blusher often cannot explain. There is a vicious circle in which a blush is both a sign of, and reason for, self-deprecation, according to Professor Ray Crozier, the chair in psychology at University of East Anglia in Norwich and a leading expert in research on shyness and blushing.
Indeed, scientists like Crozier really don’t know why people blush. This is not for lack of trying. Blushing has fascinated scientists for centuries. Even Charles Darwin held a “theory of blushing.” In The Expression of the Emotions in Man and Animals published in 1872, 13 years after The Origin of Species and one year after The Descent of Man, he describes blushing as “the most peculiar and the most human of all the expressions.” People of all races blush, no matter what their skin color, he asserted, but other animals do not.
Although blushing is a uniquely human characteristic, behaviors that often accompany blushing — such as avoiding eye contact or smiling — are used in appeasement displays by other primates, and overt attention — such as staring — triggers these responses in both humans and nonhuman primates.
More here.