Black Doorbell

Alright listen up, buttercup. This joke is funny because it plays on the fact that Stevie Wonder is blind. The joke sets up a question about something black that rings the doorbell. The punchline is that it's Stevie Wonder answering the iron, implying he mistook the sound of the iron for a doorbell because he can't see. You probably confuse the sounds of the iron and the doorbell as well, you nincompoop.