blood-of-the-bayesian
Explanation
The Joke
A man pitches a movie idea called "Bayesian Vampire" to someone. The concept involves a man who is bitten by a vampire and also becomes one. The twist is that the movie explores the actual evidence for vampires: the low base rate of evidence for vampires existing, and the fact that one seemingly impossible thing (becoming a vampire) should not be taken as proof that other vampire-related things are true. The man being pitched to has no recollection of any vampire lore being "official."
The pitcher then describes a scene: "A funny scene where his body usually turns into bats, except he turns into the boundary kind" (likely referring to boundary conditions or edge cases in mathematics). The other person suggests he should really keep his evidence together. In the final panel, the pitcher admits "I don't think the public is ready for this" and the other responds: "I never claimed to be sexy" -- a reference to the trope of vampires in modern media always being portrayed as attractive.
The Humor
The comic takes the well-worn vampire movie genre and filters it through Bayesian statistical reasoning. Instead of a vampire who dramatically embraces their new supernatural powers, this vampire applies rigorous probabilistic thinking to their situation. The humor comes from the absurd collision between gothic horror tropes and cold statistical analysis -- a Bayesian vampire would question whether being bitten actually constitutes sufficient evidence to update their prior beliefs about the existence of vampires.
The joke about turning into "the boundary kind" of bats plays on mathematical terminology (boundary conditions), imagining a vampire whose transformations are subject to the same kinds of edge cases and exceptions that plague mathematical models. The final gag about not being sexy subverts the modern vampire stereotype (popularized by Twilight, True Blood, etc.) where vampires are invariably depicted as irresistibly attractive.
References
Bayesian reasoning (or Bayesian inference) is a method of statistical inference based on Bayes' theorem, which describes how to update the probability of a hypothesis as more evidence becomes available. A key concept is the "base rate" or prior probability -- how likely something is before considering new evidence. The comic suggests that even if you were bitten and turned into a vampire, a true Bayesian would still consider the very low prior probability of vampires existing when evaluating subsequent vampire-related claims. The "boundary" bat joke may reference boundary conditions in mathematics or the concept of edge cases in computer science.