A black wart is medically known as melanocytic nevus. Symptoms of cancerous melanocytic nevus generally include sudden increase in diameter, abnormal change in color and abnormal secretion, etc. When the above symptoms appear, patients are advised to go to the hospital for examination in time. 1. Change in diameter: if the diameter of melanocytic nevus increases suddenly within a short period of time, it is necessary to pay attention to whether it has the tendency to become cancerous. 2. Abnormal color change: if the color of melanocytic nevus changes within a short time, such as red, blue or even purple, it should be observed vigilantly. 3. Abnormal secretion: If there is bloody secretion on the surface of melanocytic nevus or accompanied by itching, it may be cancerous. If the black wart suddenly appears the above symptoms in a short period of time, it is recommended that the patient go to the hospital for examination in time, and carry out the treatment under the guidance of the doctor, and do not use the medicine by oneself.