Hi,
The skin in the regions you mention is normally dark as compared to skin elsewhere.
Do you have any other associated symptoms of itching, rash , etc.
It would not be advisable to scrub the area too hard as this would irritate the skin. You should try to avoid sweat collection in the areas, keep the areas clean, maintain good personal hygiene, wear lose comfortable clothes. Apply calamine lotion at the sites.
Consult a skin specialist for your complaints and discuss whether this is normal or whether a demyleininzing agent would help in your case.
Let us know about how you are doing and if you have any other doubts.
Post us about what your doctor advises.
Regards.
When you tan do you also get white patches on your skin that refuse to tan??