It sounds like hyper pigmentation.
This is often due to sluggish veins in the legs.
Ultrasound could show this.
A lab test called CMP should be done..to make sure there is no indication of adrenal disease.
Former sunburn or any injury to the skin can result in this, including the past use of chemical creams or other environmental exposures.
The most common cause would be sun exposure, or tanning beds...why only in this area would not be clear...genetic factors can be at work.
So this is not an emergency, but I would show it to your doc this week or next, mainly for the lab, and to consider ultrasound.
Use reply if you need to clarify
Please complete the question and answer process as prompted: Then come back as needed. Positive/excellent feedback the best! Future questions? Ask for DrThomas in the question title or see my profile to request me. Save my profile to your browser favorites http://www.justanswer.com/medical/expert-dr-thomas