Thanks for the query.
Probably you are suffering from atopic dermatitis
Folds of body are prone to develop this.
You are taking a correct treatment.
The dark marks can be removed by applying a depigmenting cream like hyde cream.
Please meet a dermatologist
for exact diagnosis.
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.