Not only Black Widow spiders, but other dangers may lurk in crawl spaces, depending on the part of the country you are in. Namely, snakes, scorpions, rodents, broken glass, sharp pieces of cut off siding and flashing, etc. Wear protective gloves, knee pads, carry a flashlight and examine the area above your head space for protruding nails, screws and other stuff you don’t want to be jabbed with.
I’d also take a pinpointer and small container to clear out a lot of potential nails and metal bits that might mask a good target.
Good luck