The darkening of skin can be multiple causes like tanning,
vitamin B12 deficiency, addisonian pigmentation, acanthosis nigricans, lichen planus pigmentosus, rhiel's melasnosis or several other causes. A proper assessment and examination will help ascertain the proper cause. Kindly visit a qualified dermatologist for better evaluation.
Hope this was helpful.
Next Steps
Meet a qualified Dermatologist;