Neither. There is a network of blood vessels under the eyes called the infraorbital venous plexus. These blood vessels drain into the area of the nose, and when the nose becomes congested, the flow backs up, causing distention. These blood vessels are veins, which carry darker, deoxygenated blood. The result is a darker color under the eyes, which almost appears like a bruise. This is also known as an "allergic shiner" because of its similarity to a black eye. It is sometimes accompanied by mild swelling. The most common causes are allergies or chronic sinusitis, and treatment of these conditions will often result in resolution of the discoloration.

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