The darkening of the skin on both sides of the neck should alert you to the possibility of melanosis. Nigrosis, also known as Rell’s disease, is a manifestation of contact dermatitis. The disease can occur at any age.
The cause of the disease is currently unknown and may be a sensitive reaction of the skin in sun-exposed areas such as the neck, behind the ears, forehead and cheekbones to over-stimulation by certain chemicals (e.g. crude cosmetics) and/or ultraviolet light, which in turn leads to a darkening and blackening of the skin.