Acanthosis nigricans (AN) is a skin condition that causes one or more areas of skin to darken and thicken. The affected skin often feels velvety.
People who get this skin condition sometimes mistakenly believe that a dye or something else they touched has discolored their skin. Vigorous scrubbing will not get rid of the darkened skin.
AN is not contagious. It is not harmful, although it can be a warning sign of a health problem that requires medical attention. For this reason, it is important to see a dermatologist if you notice an area of darker, thicker skin.
Visit: https://www.aad.org/public/diseases/color-problems/acanthosis-nigricans for more information.