The area between the legs with dark color is a skin condition named Acanthosis Nigricans and occurs among people with dark skin as well as obese people.
This issue is not dangerous but it may indicate some serious disease. Scientists are not familiar with what causes this, but they say that it often occurs among people who are diagnosed with type 2 diabetes and obese people. This condition can also be a warning sign of a tumor, hormonal imbalance, and genetic factor, but rarely.
This issue can affect any part of the body not only the pubic area. The skin turns brown, and becomes softer and thicker than usual. Even though it is not painful, the condition may sometimes cause irritation and itching. For that reason, if you notice any of those symptoms, pay a visit to you dermatologist.