You might mistake it for a tan or brown stain and try to scrub it off. But it won’t work. This condition is common in people with diabetes. Your skin usually darkens and thickens, and it might feel velvety. It could itch and smell, too. The back of the neck, groin, folds of elbows, knees, knuckles, and armpits are common spots. Being overweight or obese ups your chances for acanthosis nigricans, so shedding pounds can help get rid of it.