I applied hydraquinone 4% to my dark underarms: they have turned somewhat more dark. (Photos)

Hyperpigmentation from skin to skin contact can be deep. I would
advise a hyperpigmentation protocol that includes light fractional
laser, light chemical peels, Rapid exfoliation protocols, and topical
creams. In our office, we use these treatments to lighten discoloration
of the skin. Depending on the depth of treatment, it may take weeks to
months. 
Best, 
Dr. Karamanoukian
Realself100 Surgeon