Sounds like a condition called 'Acanthosis Nigricans' which is caused by insulin resistance. In some overweight people the cells of the body become resistant to insulin and the excess spills over into the skin causing the darkening you described -- between the thighs, under the breasts, in the folds of arms, around the neck and other parts of the body that rub together. It is believed that losing weight will cure this. Cutting down on carbohydrates and exercising for 50 minutes per day, should reverse this condition. It takes time and effort, but you'll also be preventing diabetes and cardiovascular disease in the process.