What might cause temporary blurriness in peripheral vision?

Many things!: Changes in vision, blurriness, blind spots, halos around lights, or dimness of vision should always be evaluated by a medical professional. Such changes may represent an eye disease, aging, eye injury, or a condition like diabetes that affects many organs in your body. Other potential causes of vision problems include fatigue, overexposure to the outdoors and certain medications.

Many things: Migraine, impending retinal vascular occlusion, giant cell arteritis, optic disc drusen, papilledema, etc. You need to see an eye doctor.