I'm so sorry to hear about your kitty! I have a diabetic cat, too, and worry about him.
I used to have a blind cat. She could maybe see the difference between the white bathtub and the surrounding area, or big contrasts, but other than that, she was blind as a bat. She got along amazingly well (when I first got her, I put her at the bottom of the bed to sleep--because I was allergic to cats at the time--and in the morning, I woke up, and she was at the top of the bed, lying on her side, with her head on the pillow and her top paw on my arm!). She could sniff out food whereever it was. And when I visited my parents, and took her along, she amazingly found the litter box in the basement and went down and up again by herself (I had brought her own litter box so she wouldn't have to do that, but she did well on her own).
Blindness isn't a death sentence, so that shouldn't be a concern. It may take him a while to adapt if he suddenly went blind, but just make sure his litter box is in an area that's easy for him to find, and make sure there are clear paths for him to get around.
I would be concerned about theh seizures, though. And if they continue, that doesn't sound like a good way to live.