Blushing can definitely be a sign of physical attraction. Zoologist and author Desmond Morris believes that the reason that women started wearing blush on their cheeks in the first place was to represent the natural sexual flush.
However, other things cause blushing as well, and it should not, by itself, be taken as a definite indicator of attraction. According to Britain's National Health Service, blushing is caused by muscles in the blood vessels in the face that are controlled by the autonomic nervous system.
The autonomic nervous system is not something that people consciously control. When autonomic nervous system is overtaxed, it causes a slight swelling in these muscles and a slight reddening of the skin.
There are many possible triggers for the autonomic nervous system, including emotional and physical stimuli. Some diseases, such as rosacea, can cause the long-term appearance of blushing.
There is even a condition called erythrophobia, or the fear of blushing, which causes some individuals to blush even more in situations where they might blush. In addition to medical conditions and physical attraction, blushing can be triggered by a variety of strong emotions including guilt, anger or embarrassment.
It can also be caused by excitement of a non-sexual nature. Beyond emotional triggers, heat, rapid changes in temperature, exercise and exertion can cause blushing. Certain foods can also trigger blushing. The list includes spicy foods, alcohol, hot drinks and anything that contains monosodium glutamate, or MSG, a common flavor enhancer in food.