While it might not be a food allergy/anaphylactic shock--they usually occur within minutes to an hour after ingesting the suspected food--she COULD have a food intolerance to black beans, the seasonings on them, or something else she ate. A food intolerance can take up to three to four days to "make itself known"--which then makes it hard to track back to discover what it is you ate that made you sick.
That happened to me when I was 10 years old. I was at a friend's house for a sleepover and ate peppermint Lifesavers candies. A few hours after I got home the next day, I came down with hives all over my body--huge ones. The only thing we could think of that I had eaten that I normally didn't eat was the peppermint candies. A few years later, I ate some spearmint Lifesavers candies. Again came down with hives.
Figured out I was allergic to anything in the mint family--peppermint, spearmint, wintergreen, Creme de menthe, etc. In fact, if I eat any Creme de menthe cake at Christmas time I can "feel" when I've eaten my limit of cake/when eating any more will trigger the hives. Yes, I'm naughty and eat my cake too, but I can tell when I've eaten enough/when my body has had it's limit. I also know that that's bad, bad, bad of me! LOL I shouldn't tempt myself that way. Luckily, no one in my family makes creme de menthe cake anymore (I think they've figured out that I have no will power against it...)
I know about food intolerances very well because I have a whole bunch of them--plus I have an allergy to casein (a protein in cow's milk). However, there are also different forms of allergy--my allergy doesn't make my throat swell shut, nor does it require an ER visit.
If you want to know for sure, visit a naturopathic doctor and have him run some food allergy and food intolerance tests. You'll have your answers almost right away.