Most dogs and cats who lose their sight actually adjust quite well and get along fine with just a little care and assistance. Like always leaving their stuff, like food and water bowls, in the same place.
I agree with Bonestructure. A lot of older dogs can get cateracts in one or both eyes, and as long as you don't change anything in the house ( like moving water and food dishes, or furniture, etc), they can adjust quite well. Blindness can happen at any age, and I would highly recommend a vet visit to find out what is causing the blindness, and then go from there.