[roywiggins]

The first one that comes to mind is the arrival of the Conquistadors, from the point of view of the locals. Even in retrospect, could they have any reason to believe it was possible? Well, not really, no. Invasion generally, maybe, but not from another hemisphere.

It has to be something that's just not accounted for in your worldview, something you couldn't expect based on anything you know about. A meteorite wouldn't be a black swan for us, but I think it would be for a society that hadn't discovered them yet. A meteorite strike on London circa 1800 would have been a black swan, because at the time people (Europeans, anyway) thought that shooting stars were atmospheric phenomena. Large rocks falling from the sky just wasn't in their cosmology.

https://en.wikipedia.org/wiki/Meteoroid#History