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Had this conversation with someone who chose to no longer be at my table after meeting a blind NPC
(files.catbox.moe)
Another player who was at the table during the incident sent me this meme after the problem player in question (they had a history) left the group chat.
Felt like sharing it here because I'm sure more people should keep this kind of thing in mind.
5e "blindness" probably assumes blindness from a curse or spell on otherwise functional eyes since that's how I've seen the condition being afflicted. As you mentioned, losing a limb is a different thing so if they lost their eyes, had their eyes physically destroyed in some way, or were born with non-functional eyes I would rule it as the latter case at my table in those instances.
If someone is born without legs, it might take True Polymorph to add some.