what color is the rest of your body? if you are naturally dark skinned,you may just be more likely to get dark in certain areas, i wouln't worry too much about it.
I agree with dianabarry. your skin is just a different color in those areas. in fact it is not uncommon. nothing to worry about. have you heard the saying "it's black where the sun don't shine".
I also have what you have but I just didnt care too much. my best friend has it too she sometimes complains, dont worry I dont think guys notice anyways.
Dirty (?) *** 
