It can be Acanthosis Nigricans. Which means Dark skin in the skin folds like thighs, arm pits, neck, etc.,
The causes can be obesity, drugs like estrogen hormones, oral contraceptive pills, steroids, PCOD, hypothyroidism etc.,
The cause needs to be find out to prevent further pigmentation of the skin.
Next Steps
consult me online/chat for proper diagnosis and further treatment
Health Tips
DON'T APPLY any cream/ointment by yourself or from medical store guy(as they might contain topical steroids) which will sometimes increase the disease and cause complications damaging the skin