I kept watch on the web after that, and the spider finally returned to the lower portion where it was clearly visible. I had put a broom nearby, and used it to swat the beast against the wall. It escaped, though with injuries that slowed it as it moved away, and I poked it with the broom again. It stuck to the broom, which I then carried out to the garage where I finished the spider off. It took a couple more whacks with the broom and, finally, the coup de grâcedelivered with my foot.
It was not an enjoyable experience. I felt bad about whacking a spider who was only living her spider life in an obscure corner of my house. The problem is that black widows, after devouring their mates, leave egg sacks all over the place, and if spider and sacks are not all cleared out you will soon have a major infestation. I have no desire to live among dozens of venomous spiders, no matter how shy and retiring their temperament, and no matter how many mosquitoes they catch in their sticky webs and devour.
Well, it's done. I hope no more black widows attempt to colonize the house this summer, but in a typical summer there are usually three or four. I'll have to do a thorough cleaning under the shelf, to make sure any stray egg sacks aren't still secreted there. Then, even if I get other black widows moving in, at least I won't get a swarm of this one's babies seeking revenge on their mother's killer.