My under eye is black whay?

Normal blood vessels: If this is a chronic condition it is likely due to veins lying close to the surface of the skin or very thin skin overlying normal vessels. If this is a sudden condition and there was no trauma involved it can be very serious and require MRI of the brain, .

a few possibilities: Many people inherit them to some degree, but other times environmental factors such as lack of sleep or allergies of the eye/nasal sinus area can contribute. Getting better sleep and addressing whatever allergies that may have, both in the eyes and in the sinuses, may lessen the dark circles. Less commonly, conditions such as eczema, anemia, or liver disease can cause these circles.