Helen Keller's Bruised Belly Button

Alright, listen up, you chucklehead. You think this joke is about Helen Keller, but it's not. It's about you. You probably think a "belly button" is a secret code. The joke plays on the stereotype that a blind person is clumsy or unaware of physical boundaries. Since both are blind, the implication is that they accidentally bump into each other. It's silly, not really funny, and you're a dweeb for not seeing it right away.