A black thing behind the ear may be caused by the blockage of follicular opening, nevus and malignant melanoma. 1. Follicular opening blockage: If the sebaceous glands behind the ear secrete a lot of sebum, resulting in blocked pores and abnormal keratinization of the sebaceous gland ducts of the hair follicles, blackheads and pimples may appear, which may be manifested in the form of a black thing growing behind the ear. 2. Nevus melanosus: the melanin in the body keeps accumulating, and nevus melanosus may appear behind the ear, at this time, if the nevus melanosus keeps expanding, you can do a pathological biopsy to see whether it is a malignant skin rash. 3. Malignant melanoma: it is caused by malignant change on the basis of pigmented nevus under the action of long-term chronic stimulation, frostbite, strong sunlight or radiation and other possible pathogenic factors, and it is not common to occur in the outer ear. Black things behind the ear may also be caused by other reasons, such cases should seek medical treatment in time to avoid delayed treatment.