Skin discolouration. Weight for height.

I am a 23 year old female,I have no past or present history of any diseases.But as I reached my teenage I started gaining weight ,and I did all kinds of hormonal and blood profile check up for weight gain since my parent's compulsion.But the weight gain was because of over eating I did.After I started gaining weight ,I noticed that that my groin and vulval region had become darkish black to purple colour,I have this discolouration only on these areas and nowhere else ,because I am very fair,this area alone stands out and causes me emberasment .I consulted a dermatologist ,he advised me to apply calmoseptine.What could be the cause for this discolouration?I have started loosing my weight ,I am down by 10 KGS with proper diet and work out?will this darkness vanish completely after I reach my ideal weight for height.My current weight is 70 kg,I am 162 cm,No one else in my family has this problem or diabetes