Dark skin pigmentation may be due to a lot of factors. However, you have already mentioned that you have no weight problems and are not diabetic. For some, skin pigmentation may be genetic as well as hormonal.
I suggest that you have a work up done on this. I would still suggest this since we do need an objective baseline examination to ascertain if diabetes is present Conditions involving the adrenal glands should also be ruled out. At this point, you may apply Vitamin E cream over the area at least thrice a day. This has skin softening and lightening qualities. A gentle Vitamin C exfoliant may also be used to remove dead skin which may also contribute to the dark color. Avoid prolonged sun exposure. Papaya, licorice and cucumber are some natural ingredients that have skin lightening properties.