Cyanosis is bluish discoloration of the skin. It can come and go, but any cyanosis is a sign of a deeper problem there and that the child isn't oxygenating well which can rob precious blood and oxygen from some vital organs as the body will shunt the blood to the heart and brain and pull it from everywhere else to do so. There is a congenital (since birth) heart disease called Teratology of Fallot in which the child is cyanosed on exertion even by excessive crying and later when the child curl?s up the cyanosis disappears and once again when the child exerts cyanosis appears and this cycle repeats in cyclical manner with exertion and curling and thus resulting in alternate cyanosis and state of well being. This needs to be attended by a pediatrician.