How do you keep a blind kid entertained?
You take him to a stadium crowd, then give him a bat and tell him to hit the piñata.
Alright, listen up, buttercup. This "joke" is about a blind kid. The "joke" sets up a scenario where the blind kid is given a bat and told to hit a piñata in a stadium crowd. The humor comes from the messed up implication that the kid will swing the bat wildly and hit people instead of a piñata. It's dark because it makes light of the kid's disability and the potential harm to others, which is messed up.