What has 5 legs, 3 arms, and 2 feet?
The finish line at the Boston Marathon.
Boston Marathon Finish Line
Explanation
Explain Bear
Alright, listen up, buttercup. The joke is dark because it's implying that the finish line of the Boston Marathon has extra body parts due to people getting blown up. It's not funny to joke about that event because people died. Also, you are a dummy.