I thought it may be possible that because he was one of the first to look at the sky through lens, he was unaware that looking directly at the sun without any filter would cause blindness.
Almost certainly not, he retained good vision for a couple of decades following his use of a telescope to directly observe the sun. We know he lost vision after his 70th year, so the differential diagnosis would most likely be: 1 - age-related macular degeneration (frequently causes blindness in one eye long before the other; 2 - cataract; 3 - glaucoma.