The most likely reason for a darkened skin around the neck is that you have tanned the area quite badly over a period of time, due to exposure to the sun. This is a very common and normal occurrence, which is also the body’s reaction to exposure to the harmful effects of ultraviolet radiation. However, there is a more serious cause that should also be explored, which is the development of type 2 diabetes. Type 2 diabetes is a condition in the body, where the insulin that is produced in the body is no longer effective at its task of reducing blood sugar levels. Thus the body becomes resistant to the effects of insulin. You should go to an endocrinologist to get your serum glucose levels checked.
The condition is called acanthosis nigricanis and is a result of insulin resistance. When there is a resistance to insulin in the body, the levels of insulin that is circulating in the blood increases. This insulin affects the pigment creating cells of the body called melanocytes. The melanocytes then create too much melanin – the skin pigment – which, when exposed the sun, darkens. The typical areas of affliction are the back of the neck, underarms, and groin. You should get on the internet, look for images of the skin that is affected by acanthosis nigricanis, and compare it to your condition. Type 2 diabetes usually caused by obesity and this kind of problem is also a typical sign of obesity. If this is not the reason that is causing the darkening, then you have just tanned the area too much. The back of the neck is a much-neglected zone when it comes to protection from the sun. When skin is exposed to the ultraviolet rays of the sun, the melanin in the skin gets a bit darker because of the oxidative effect that ultraviolet light has. This mechanism also protects us from the harmful effects of radiation, as the oxidative reaction also converts ultraviolet light into harmless heat. When there is too much exposure to the sun, the skin also ends up creating more melanin to cope with the exposure to the sun and this becomes a cascading cycle of further tanning. This is usually irreversible after a certain point, as the tan becomes permanent.
Relieving yourself of this situation would require you to start bleaching and exfoliating the skin in the back of the neck. This can be done by using nature’s own bleaching agent vitamin C. Use a mixture of squeezed lemons and onions and apply it to the back of the neck. Measure the improvements weekly.