Darkening of the neck warns of four diseases that are not real. There are many reasons for the darkening of the neck, such as acanthosis nigricans, lack of attention to hygiene, sun exposure, and hyperpigmentation. 1. Acanthosis nigricans: and low immunity, obesity, drug stimulation, may lead to black neck, some will also cause itching, skin with papillary redundancy. 2. Not pay attention to hygiene: usually do not pay attention to personal hygiene, no timely bathing, no cleaning secretions, due to sweat, dust and other stimuli caused by local skin dirt, can also cause the neck black. 3. Sun exposure: ultraviolet radiation can cause subcutaneous melanin secretion, can also cause the phenomenon of neck blackening. 4. Hyperpigmentation: neck skin eczema, infections and other diseases, after the recovery period, may also cause hyperpigmentation, causing neck blackening. There may be other reasons for the darkening of the neck, it is recommended to go to the hospital for examination and treatment as prescribed by the doctor.