Blushing is the reddening of a person's face due embarrassment, anger, or romantic stimulation.
When we are in a state of embarrassment, anger, or romantic stimulation a hormone called Adrenaline is released. This also triggers the fight-or-flight response, and the resulting action of adrenaline causes blood vessels to increase in diameter along with an increased blood supply to our facial skin. This entire reaction is controlled by the sympathetic nervous system and is completely involuntary and cant be controlled.
There are generally more blood capillaries and more blood vessels in our facial skin (cheek) as compared to other skin areas. Furthermore, this widen blood vessels in the cheek even more, since they are already wider than most other blood vessels and also happen to be nearer the surface of our skin.So blood flow wll be high causing reddening of cheeks