The softer stuff is the same as the dried black stuff before it dries. As Oscar is blind he may blink less than sighted cats, I'm wondering if this causes his eye bogeys to form more readily? If he had a cold he would show other symptoms.
I think RosellaMoggy's boy Billie Wizz has eye trouble this time of year, but that seems to be pollen allergy related, maybe she has some knowledge?
I think you're the only Purrs owner with a totally blind cat at the moment, Princess. There may be some eye drops you could get from the vet to "wet" his eyes a little? I know they make them for humans suffering dry-eye but it would never be appropriate to use human remedies on a cat without your vet's approval, and there may be an animal specific remedy.