I once used a cream called fa for shaving, my skin in the neck and chin irreted and turned into black, what can i do to remove the black particles?

FA Shaving Creme: Fa shaving creme is licensed under a number of manufacturers and like many grooming products, contains a variety of proprietary additives one of which resulted in your skin condition. Often the pigmentation will respond to bleaching agents that your dermatologist can prescribe. I would definitely make an appointment with a dermatologist rather than trying any self remedies.