Black-backed Oriole is rather closely related to the ABA area-breeding Baltimore and Bullock's Orioles. I don't know much about Black-backed Oriole, but from some online image searching it would appear that females and young males are not dissimilar in appearance from those age classes of our more familiar orioles. Check out some of those eBird checklists with images of the species in Mexico and you'll see what I mean. It would not take much for, say, a few young birds to have slipped through the cracks before now. That age class is more prone to wander. Adult males, less likely to wander...but they are the plumage class that would stand out, like the PA bird. Something to consider.
Here are a few images - really crushed it! :)
|adult male Black-backed Oriole|