It could be a number of things with regard to his use of the tray, especially with him being blind too.
I would say perhaps try a second and even a third tray with different litters in it to that which you have in the original tray.
Also think about the size of the tray - if it's too small, that may be contributing to the issue. A larger tray or a shallower tray may be worth investigating.
As he's defecating outside the box, it's not being covered up, so maybe scent is playing a part here - given he cant see, he will rely much more on his other senses to feel safe and confident in his surroundings.
There's a super book (which might be hard to get hold of, but worth a look on Amazon) by Nicholas Dodman called The Cat Who Cried for Help by a British Vet who works in the US, and there's a whole chapter in that about inappropriate toiletting and the issues which can arise and how to help work out what's causing it.