… the most peculiar and most human of all expressions." - according to Charles Darwin.
Blushing is the temporary reddening of a person's face due to (apparently specialised) blood vessel dilation mechanisms. It is normally involuntary and triggered by emotional factors, such as those associated with embarrassment, anger, or romantic stimulation etc..
It has been suggested that blushing and flushing could be visible manifestations of a physiological rebound of the basic instinctual fight/flight mechanism, when physical action is not possible.
However, no exact mechanism or evolutionary advantage has been determined.
More info : wikipedia