blue spot, 1. (Informal) one of a number of small grayish blue spots that may appear near the armpits or around the groins of individuals infested with lice, such as in pediculosis corporis and pediculosis pubis. These spots are usually less than 1 cm in diameter and are caused by a substance in the saliva of lice that converts bilirubin to biliverdin. Also called macula cerulea. 2. (Nontechnical) one of a number of dark blue round or oval spots that may appear as a congenital condition in the sacral regions of certain children less than 4 or 5 years of age. They usually disappear spontaneously as the affected individual matures. Also called Mongolian spot.
