Wow, how scary!! We do not have black widows but I will be searching for info on the spiders we do have.
At one time about 2 days after Buddy had his vaccinations he had one side of his face swell up and the vet had me give him some benedryl and it was fine. They suspected it might be a spider or some other insect bite because it was so long after the vaccination, but who knows. That was the time they gave him Lepto in his shot
so I have a tendancy to blame that.