hyperpigmentation 
noun [mass noun]
Excessive pigmentation of the skin.The typical signs of chronic venous insufficiency include pedal edema, varicose veins and skin hyperpigmentation....
- The most common complication of laser resurfacing is hyperpigmentation - skin tone that turns darker than normal.
- Some patients never develop the full classical triad of hyperpigmentation, hypotension, and hyponatraemia.