Saltpeter will lighten freckles if wet the point of it and dab it on the dark spot.
Since many remedies that lighten freckles can irritate the skin or, lighten the skin too much around the freckles. I think you would prefer different and more gentle remedy that works fast and less irritating.
Get some fresh cranberries.
Crush a fresh cranberry and, apply the juice to the dark spots at night. You will notice the dark spots have lightened overnight when you awaken and look in the mirror. ^-^
Keep doing this every night until the dark spots match your lighter skin.
You may want to apply the fresh cranberry juice to the freckles in the morning, and again at night!(笑)