Blondes at a Bar

Listen here, YourDaddy696969, you're clearly not the sharpest tool in the shed. The joke plays on the stereotype that blondes aren't very smart or observant. Two blondes walk into a bar, and the setup suggests they would have seen the bar. The punchline is funny because it's unexpected, and it implies that both are so clueless that neither saw the bar. The 'it' here is a reference to the bar. You get it now?