When the black widow bites they're in complete control of how much, if any, venom will be injected. Bites can be inconsequential, painful or fatal, it's all up to the spider, how big it is, it's diet etc. The bite of a black widow can be positively identified by examination with a magnifying glass. If positive identification is made statdard treatment is calcium gluconate and sometimes convulted serum. There is evidence to suggest the addiction of ascorbate to the gluconate is much more efficacious; here's a paper from the 50s with a case study.