Just apply Face Mask by Jupiter Beauty, its a cold cream like, pure natural, no chemicals so no harm will be done. Once the cream dries 10mins , just wash and you will se no dark spots.
I once had a purple discoloration on my lip due to long term antibiotic use and oddly enough I inadvertently found that using whitening toothpaste and mouthwash got rid of it over time because of the way it was always so drying to my lips (I guess we could call it exfoliating because eventually, it sorta just peeled away).