It can be sun exposure or hyperpigmentation skin that is causing such darkening. Avoid going out in the sun during the peak hours and apply a sunscreen with minimum spf 30 atleast half an hour before going out in the sun. Also dermabrasion, laser resurfacing, chemical peeling, and microdermabrasion can be used for smooth and supple skin. Please consult a dermatologist for that.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.