Maybe. Depending on your definition of loss of vision, and whether you are talking about an eye allergy or a systemic allergy, it is possible. Blurred vision is not uncommon in eye allergies, and also when treating allergies with oral antihistamines. A systemic allergic reaction should not cause vision loss (darkened vision). This should be immediately evaluated by an eye doctor.
Probably not. Usually allergies cause watery and itchy eyes - most often blurring. Anaphylaxis is an entirely different story - this is the most extreme reaction and can be life threatening - this is something to go to the emergency department for.