What can cause temporary blindness in one eye? The patient is 86 and has cataracks. She refuses surgery.

Several concerns. Transient unilateral blindness may be seen in "amaurosis fugax", due to decreased blood flow in ophthalmic artery which may be associated with carotid stenosis. The visual issues may well be an early sign of future major stroke risk and must be heeded. An embolism from heart to the similar vascular bed is another troubling cause. Headache and visual loss may be seen in temporal arteritis.
A long list. Do you mean the eye went blind and has now recovered? Or has one eye gone blind and stayed that way? Cataracts cause slowly diminishing vision in the affected eye (s). Sudden loss can come from vascular accidents, central nervous system changes, acute glaucoma, arteritis, intraocular bleeding, optic neuropathy and so on. Some of these can affect both eyes. She needs an ophthalmology exam.
Temporary . Monocular blindness can be an early warning of a stroke called a tia. See a doctor immediately.