Dark skin on the neck can be caused by acanthosis nigricans, neurodermatitis, fungal infections and so on. 1. Acanthosis nigricans: the cause of the disease is not very clear, and genetic factors, drug factors, certain disease stimulation has a relationship, may cause the skin to deepen, the appearance of the skin gray, light brown, the surface of the rough and dry appearance. It is possible to cause black skin on the neck. 2. Neurodermatitis: the etiology is unknown, and neurological factors scratching stimulation. It may cause mossy skin with itching and hyperpigmentation, leading to the appearance of dark skin on the neck. 3. Fungal infections: Fungal infections such as lichen planus and ringworm may also cause the appearance of dark skin on the neck. There may be other reasons for the black skin on the neck, it is recommended to go to the hospital in time to check, clarify the cause of the disease, and treat the symptoms under the guidance of the doctor.