If skin at the site of dark patch has similar texture and contour then there is nothing to worry .
Colour has changed to slight brown & slightly light pinkish as seen in photo.
There is likely reason for this pigmentation.. like frequent friction or pigmentation secondary to other primary skin lesions.
Next Steps
Go for live clinical inspection examination of patch , that will help a dermatologist to know probable cause of this pigmentation.
Otherwise pigmentation is treatable nothing to worry