If we're talking strictly about what's on the page and screen, I can't recall a single panel or episode where they confirm a specific congenital condition. The manga panels that show his childhood show his eyes are already shut and scarred, but that could mean a lot of things. The 'Demon Slayer' wiki says born blind, but wikis are edited by fans and sometimes extrapolate from visual hints. I lean towards him being blinded very young, maybe by an illness or the same trauma that killed his family, because his senses are so hyper-developed in a way that feels like a lifetime of compensation rather than something innate. His fighting style, the 'Stone Breathing', is all about feeling vibrations through the earth – that seems like a skill forged from necessity over decades, not something a seeing person could just switch to after an adult injury.
That said, the ambiguity might be intentional. His backstory is so profoundly sad, focusing on the loss of his adopted family and his immense guilt, that the exact mechanism of his blindness almost doesn't matter. It's part of his tragic fabric. The fact that he refers to others by their 'sound' and 'smell' is so ingrained, it feels foundational to his entire being. I think Gotoge left it vague because the emotional truth—that he perceives the world through a different, profoundly empathetic lens—is far more important than a medical chart detail.