Periorbital Ecchymosis (Black Eye): This is the most likely diagnosis given the sudden onset and localized nature of the darkening around the eye. It is often caused by trauma, which could be minor and not necessarily remembered, leading to bruising.

Allergic Reaction: An allergic reaction could cause swelling and discoloration around the eye, especially if it's part of a larger reaction that includes the face.
Insect Bite or Sting: Certain insect bites or stings can cause localized swelling and darkening of the skin.
Contact Dermatitis: Irritation from contact with certain substances can lead to skin darkening and inflammation around the eye.

Basilar Skull Fracture: Although less common, a basilar skull fracture can cause periorbital ecchymosis (raccoon eyes) and is a critical diagnosis not to miss due to its potential for serious complications.
Orbital Cellulitis: An infection of the tissues surrounding the eye, which can cause redness, swelling, and discoloration. It requires prompt antibiotic treatment to prevent serious complications.
Cavernous Sinus Thrombosis: A rare but potentially life-threatening condition that can cause periorbital edema and discoloration, often accompanied by other neurological symptoms.

Amyloidosis: A condition characterized by the deposition of amyloid proteins in various tissues, which can rarely cause periorbital purpura (bruising).
Vasculitis: Certain types of vasculitis, like granulomatosis with polyangiitis, can cause a variety of symptoms including skin discoloration around the eyes, though this would typically be part of a more systemic illness.
Malignancy: Very rarely, darkening around the eye could be a sign of a malignant process, such as a metastatic tumor affecting the skin around the eye.