How does botulinum toxin enter the blood stream from the digestive tract? To my understanding, large polypeptides such as botulinum toxin cannot pass the intestinal lining intact. How, then, can it enter the bloodstream and cause botulism poisoning?

Transcytosis is a process by which large macromolecules are transported across a cell, such as those in the intestinal epithelium. It is used by many toxins and even whole organisms to enter the body. Botulinum toxin also apparently uses this mechanism. I'm not sure how detailed of an answer you want, but the linked to-review is pretty in-depth.