The dark areas - do a search for Acanthosis nigricans and see if the pictures match what you are seeing. Here is a link - http://en.wikipedia.org/wiki/Acanthosis_nigricans.
This can be perfectly normal - I know lots of people with it. I know one who has this and have been diagnosed with PCOS (polycystic ovary syndrome) and is being treated with Metformin - same as used for Type 2 diabetes. She has lost weight and the darkness around her neck and underarms has faded.
Have you seen anyone about it? If not, make an appointment with your Health Care Provider and have this checked.