A congenital blue nevus is one type of birthmark. It is not known what causes birthmarks, and treatment is usually not required unless the mark is on the face. Blue nevi are blue in color because they are positioned deep within the skin, and appear to be a ‘bruise’. They can range in appearance from flat to slightly raised, and they are usually small in size. Some nevi will fade as the child ages, but most will remain unchanged throughout a lifetime. The presence of multiple birthmarks warrants evaluation by a physician.