If a single toenail is blackened, it may be caused by external pressure or bruising under the nail. Usually, in this case, no treatment is needed, as long as you wear loose shoes, it will recover automatically or gradually return to its original appearance after one or two months. If it is a nail nevus, the blackening is usually regular or streak-like, and usually does not require any treatment. If the nail nevus is blackened and does not have any conscious symptoms, and does not subside for a long time, and some of them even appear to be enlarged, it is best to go to the hospital for examination. Through dermoscopy, first roughly determine whether it is malignant black, if it belongs to malignant black, need timely surgical treatment.