Jimmy is blind. When Castiel takes him as a vessel, he doesn’t bother restoring his eyes, as thousands of his true form’s are enough to see everything for miles.
This is not an issue until Metatron steals his grace.
Other ideas I’ve been thinking about:
• Cas stares right through Dean in Jimmy’s body, making him believe all angels behave that way in vessels. Until he meets Uriel and realizes something is up with Cas in particular.
• When Cas is forced out of Jimmy in The Rapture, Jimmy is disoriented and terrified, realizing the angelic sight Castiel granted him is gone.
• Jimmy is still able to make it to his family on his own. He has been blind for longer than he had angelic sight, and he wasn’t going to let that loss stop him from seeing his family again.
• Jimmy is able to sense that Roger is a demon before he reveals himself, because he can smell the sulfur on him. Perhaps the same happens with Amelia later on.
• When Cas is low on grace, he is still able to see through his angelic form, just not as broadly as he can at full power.
• When his wings are damaged in the fall, a part of his true form’s eyes are as well.
• If you think Jimmy was terrified finding himself somewhere he couldn’t recognize with no sight, imagine how it would be for Cas.
• Cas can’t see the fall of the angels, but he can feel the burning grace on his face.
• He still instructs Dean to look after Sam instead of coming to get him on the phone. He doesn’t mention the blindness. He doesn’t want to burden Dean when Sam needs his whole attention.
• He barely makes it to the bunker, relying on the kindness of strangers to help him get where he needs to go. It is so much easier for April to trick him this way.
• I can’t imagine Dean kicking Cas out of the bunker when he finds out Cas is human and blind. He bargains with Gadreel to let Cas stay at least a few days to adjust. But I do think Cas would leave voluntarily if he found out there was a danger to Sam’s life, perhaps sneaking out at night.
• Cas feels incredibly compelled to still be useful despite his human disability. While at the bunker, he studies the area meticulously, maps out the entire perimeter in his mind, practices his blade skills even harder, as he is unable to learn to shoot.
• If Cas leaves, Dean goes to find him immediately. Eventually, he does, but Cas refuses to come back as long as Sam may be in danger. Dean accepts his choice, but promises to keep checking up on him regularly. He does, coming to where Cas is staying way too frequently, worrying sick. Cas struggles at first, but adapts to his disability with time, and eventually starts doing well for himself, perhaps still working the same job at Gas-N-Sip.
• Cas has a freakishly good memory of where everything is. Both a remnant of his angelic abilities and a personality trait, in my opinion. Helps that he is extremely determined to memorize the area, too.
• Another angel’s grace is able to help him manifest his true form again, but its vision, hearing, senses all feel wrong. The colours are all wrong, the vision is fuzzy and blurry and static-y all at once, the grace constantly fighting against him. He finds that closing his angelic eyes and relying on his body’s senses, as he did as a human, is way better than trying to battle against the wrongness of the grace.
• Cas never learns how to drive.
• When he gets his own grace back, at first, Cas doesn’t consider restoring his body’s eyes. As the grace regenerates enough, Cas remembers the struggle he went through when he suddenly found himself stripped of all vision. He then remembers Jimmy’s life, his family, the way he lived and worked and loved with his disability, no worse or better than any other human being. He remembers his own initial struggle, but being able to adapt to it, and embrace the limitations placed upon him. He leaves the eyes as they are.