As I understand it, it's something unpredictable and high-consequence, not necessarily negative. The origins are in Hume's problem of induction. https://en.wikipedia.org/wiki/Problem_of_induction

Unpredictable positive events usually just don't get attention. Something good just happened, okay, that's good. People just don't pay attention.

Technically black swan might not be negative, but for all practical reasons, "black swan event" is what people call impactful unpredictable negative events and they expect those events to be negative. In other words, it is a synonym for "a disaster" of sorts, only "economical".

This is how an economist would call a disaster.