Honestly, there's a good chance the mic made noise when it was hit and he could hear it falling.
...yea I bet he was getting his vocals piped into his ear, heard the mic get bumped, and could hear it moving away from him through what was piped in his ear, and got lucky when he put out his hand.
Also, even if he isn't completely blind ...would it be so crazy to think maybe he had some surgery at some point that restored "some" part of his vision after his identity was already established as the blind guy that could sing like a mfer....so maybe he decided to let it be.
IDK...I wouldn't be bent at him if he could...