The inguinal area is slightly normally darker in most individuals and could further darken due to infections, inflammations, friction injury etc. You could also try creams/ ointments containing tretinoin, hydroquinone, arbutin, azelaic acid, niacinamid etc. It is advisable to use these at night and avoid using on or near genitals. If these do not seem to help, you may consider seeing a dermatologist for alternative management.
Hope this is helpful.