I got this dark skin which start as hard skin then became normal but the color is darker !
hard to tell, but it could be anuthing from skin dryness to friction/ abrasion in that part of the skin.
It's hard to suggest anything without a pic.
This should help you add images:
https://patient.uservoice.com/knowledgebase/articles/402861-how-do-i-add-an-image-to-my-post
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