You have most likely developed Pseudoacanthosis Nigricans which is commonly seen in diabetics. The one thing that will truly help you is exercise. Regular exercise (150 min per week) will help regulate your insulin levels . This in turn will not only help decreasing your dark circles but also help control blood sugar levels . Plus, a visit to a dermatologist will help you too. Other than diabetes there are many reasons why you can have dark circles . Stress, faulty eating and sleeping habits etc can aggravate the circles. So start exercising and visit a dermatologist. You will surely benefit.