Why are the inside of my butt cheeks, inner thighs and armpits dark?

Although it's difficult to see from the photo provided, you may benefit from eye creams with blends of kojic acid, vitamin K and/or hydroquinone.  This will definitely give some remedy to the pigmentation.

Thank you for asking about your belly button darkening after your tummy tuck.First, try using 100% shea butter or cocoa butter on the belly button. Then, keep using that but also ask your surgeon to prescribe 4% hydroquinone.AND keep the belly button out fo the sun -If you expose darkened skin ...

Hyperpigmentation and discoloration on the body can only be improved with a combination of topical creams followed by laser therapy. We recommend the Kareskin Body Brightening Kit: MelaclenzMelapadsLuminase and Exfolase creamsFollowed by fractional laser, Covalent peels, and Yag laser...