Subconjunctival Hemorrhage: This is the most likely diagnosis given the sudden onset of painless redness in the left eye, described as "shocking" in appearance, and the objective finding of a subconjunctival hemorrhage on the medial aspect of the left eye. Subconjunctival hemorrhages are common and can occur spontaneously, often without a clear cause, but can also be associated with hypertension, which is present in this patient.

Conjunctivitis: Although the patient's vision is unaffected and there's no mention of discharge or itching, conjunctivitis (either viral or bacterial) could present with redness. However, the absence of these symptoms and the specific finding of a subconjunctival hemorrhage makes this less likely.
Dry Eye Syndrome: This condition can cause eye redness but typically is accompanied by symptoms of dryness, grittiness, or irritation, which are not mentioned in the patient's history.
Blepharitis: Inflammation of the eyelids can cause redness, but it usually involves the eyelid margins and is associated with symptoms like itching or burning, not described in this case.

Acute Angle-Closure Glaucoma: Although the patient's vision is unaffected and there's no mention of pain or blurred vision, which are typical symptoms, acute angle-closure glaucoma is an emergency that can present with redness and must be ruled out. The normal visual fields and lack of pain make this less likely, but it's crucial to consider due to its potential for severe vision loss.
Retinal Vein Occlusion: While more commonly associated with vision loss, retinal vein occlusion can sometimes present with sudden onset of redness. The patient's history of hypertension and diabetes increases the risk for this condition.
Endophthalmitis: A severe infection within the eye, usually following surgery or trauma. The patient does not have a history of recent eye surgery or trauma, but this condition is so serious that it must be considered in the differential diagnosis of any acute red eye.

Scleritis: Inflammation of the sclera, which can cause severe eye pain, redness, and tenderness. It's less common and usually associated with systemic autoimmune diseases.
Uveitis: Inflammation of the uvea, which can cause eye pain, redness, and sensitivity to light. It can be associated with systemic diseases or infections.
Carotid-Cavernous Fistula: An abnormal connection between the carotid artery and cavernous sinus, which can cause eye redness, protrusion, and vision changes. This condition is rare and usually associated with trauma or vascular malformations.