A cataract is an opacity of the lens, and can be slight or complete. Light will still enter and stimulate the retina, consequently the pupil will contract or dilate depending on the light stimulus.
If your pet's pupils are dilated, then there is most likely a problem with the retina. Potentionally, the pupil may be paralyzed or the optic nerve is damaged or the brain can not integrate the visual signal, but these are less likely reasons for the symptoms described.
You need to see a veterinary ophthalmologist in order to determine the cause of blindness. My two big rule-outs for this presentation would be retinal detachment secondary to hypertension or progressive retinal atrophy (though this usually shows up earlier in life). Check with your vet to find a veterinary ophthalmologist close to you.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124