Blue nevus is a benign lesion, a benign tumor formed by limited proliferation of melanocytes in the dermis of the skin, which is generally not dangerous, but there is a possibility of malignant transformation. The cause of blue nevus is unknown and may be related to the abnormal accumulation of melanocytes in the dermis during embryonic development. Ordinary blue nevus is usually present at birth, and in childhood, it appears as blue or grayish-blue nodules on the extensor side of the hands and feet, with a diameter of about 3~10 millimeters, and grows slowly, and does not become malignant in general. Cellular blue nevus occurs in the buttocks and sacrococcygeal area, and usually looks like a large and raised nodule or plaque, with a diameter of about 1~3 centimeters or larger, and has the possibility of malignant transformation. Malignant blue nevus can originate from common or cellular blue nevus, which is a type of skin melanoma. It is necessary to visit a hospital to clarify the type of blue nevus. Treatment is mainly surgical, and pathologic examination is necessary to determine the benign or malignant nature of the nevus.