It seem that you had developed Post inflammatory Hyper- pigmenation at the thigh area which left the black marks. It is a common condition mainly seen in bit overweight
persons. Do not apply any home remedies
which can aggravate the situation. You can apply creams such as Azelaic acid, glycolic acid
and kojic acid
can be used to reduce the pigmentation over the groin area under dermatological supervision. Do not apply any thing near vaginal area as it can irritate the soft skin over this area.