Thank you for submitting your question.
You may have a condition known as acanthosis nigricans. Acanthosis nigricans is a skin condition characterized by dark, thick, velvety skin in body folds and creases. Most often, acanthosis nigricans affects your armpits, groin and neck.
Acanthosis nigricans is often associated with conditions that increase your insulin level, such as type 2 diabetes or being overweight. If your insulin level is too high, the extra insulin may trigger activity in your skin cells. This may cause the characteristic skin changes. In some cases, acanthosis nigricans is inherited.
There's no specific treatment for acanthosis nigricans. Treating any underlying conditions may cause the skin changes to fade, however. If you're overweight, losing excess pounds can help.
I understand that you have already tried skin-whitening products and microdermabrasion. Retin-A is another commonly-used product to treat this condition. Fish oil supplements may also be helpful. Laser therapy is costly but can usually decrease the amount of pigmentation.
I hope this answers your question.