An extract from this website (causes of dark circles) http://beauty.iloveindia.com/basic-tips/dark-circles.html

Heredity: Dark circles under the eyes can be an inherited trait. If you have dark circles, then it is likely that other members of your family have dark circles too. The skin under the eye is very thin. When blood flows through veins close to surface of the skin it can produce a bluish tint. If your skin is transparent then the darker the circles will be. Transparent skin is also a inherited trait.

Exposure to Sun: Exposure to sun especially during the summer months can make the skin under the eyes look darker. This is true for dark skinned people as well. Suntans are caused by an increase in the natural pigmentation of the skin, as a result of exposure to the sun. This increase in pigmentation is drawn to the surface. This applies to the skin under the eyes as well.

Allergies and Eczema: Any infection of the eye that leads to itching can contribute to dark circles as rubbing and scratching the eyes darkens the skin. People who suffer from hay fever will notice that dark circles appear when the allergy is at its height. Certain food allergies can cause dark circles too.

Medication: Any medication that dilates your blood vessels can cause dark circles under the eyes. As the skin under the eye is delicate an increase in blood shows through the skin in the form of dark circles.

Nutrition: An improper diet, an unbalanced diet or lack of a nutritious diet can cause dark circles under the eye.
Tiredness and Lack of Sleep: A lack of sleep or excessive tiredness can cause the skin under the eyes to become pale, thus, making the blood flowing under it visible.

Pregnancy and Menstruation: During pregnancy and menstruation the skin becomes pale and the veins under the skin become visible.

Age: As you grow older there are greater chances that you will have dark circles and they may become more prominent and permanent. If there are too many folds in the skin under the eyes, it will make the dark circles more prominent