Apart from writing difference of books and publishing hundreds of papers on the subject (see Nassim Talib), the tl;dr is economists and sociologists still really have no way of truly accounting for them besides being cognizant of their existence.
A true black swan is something that's basically inconceivable before it occurs. It is outside the mental model used to evaluate outcome probabilities. Black swan events include things like:
---Your enemy in a war invents the nuclear bomb and drops it on your cities (this happened to Japan in 1945)
---You are a turkey. The farmer comes, as he does every day. Instead of feeding you, he slaughters you (something which you've got no experience or conception of).
---It's 1994 and you own Blockbuster Video, a solid earning company. Then someone invents the Internet...
Biological warfare attacks are quite well-known as a concept, making them gray swans. Also in the gray swan category are asteroid strikes, supervolcanoes, etc etc.
I try to spread this distinction because if we start calling gray swans black swans, that will hide from us the most important part of Taleb's argument, which is that true black swans are the most important events. The idea of the black swan is hard to keep in your mind (how do you keep in mind the inconceivable?) so it's very easy for a pretender concept to sneak in and steal the label.
It was also well-known that U.S. is the world leading scientific and industrial power, that it absorbed huge number of well-educated Jews fleeing Nazis, and it was least affected by ongoing WWII of all major powers.
So if anyone was to make a bomb it was U.S., every rational actor should have expected it to happen. Only 'if' was timing.
Wormholes, for example, have been theorized for decades. Nevertheless, "The earth is about to fall into a wormhole and end up in another dimension" would probably fall into the category of black-swan events.
This was known before it was dropped on a city. The Trinity test happened about three weeks prior to dropping the first bomb on Japan. The second bomb was dropped with full knowledge of the potential casualties after the devastation of the first.
We Americans always justify these bombs, but really, there is no justification.
So you'd have volunteered to be the first wave on the beach?
The best explanation for why the US wanted to have a swift end to the war with Japan was to deny the USSR the option of dividing Japan like was Germany.
There is no good way to justify killing civilians during the war, and it's impossible to say what would have happened if these bombs were not dropped and some other path was taken.
Arguably keeping the USSR out of the islands may well have caused less misery over time for the Japanese people, from the purely utilitarian perspective of looking at how populations under Soviet rule fared.
Earlier than that on the European front, there was Dresden, which killed about a third of as many that as the atomic bombing of Nagasaki.
Really, once the US established control of the air, they were capable of wiping out Japan's cities with conventional weapons.
The second, just by its gruesomeness factor remains a black swan, irrespective of the number of deaths.
In the same vein the nuke might not be as deadly as all the bombings before it, but at the same time it was one new weapon (ie. a singular drop from a single plane) that did something totally new and horrific, thereby making it a black swan.
Good to know!
And you're right, you basically can't predict things like this -- but a great many catastrophic risks that can be prevented are ignored because the event seems much less likely than it is due to the scale of the impact being so large as to be unimaginable by many. Climate change is a chief example; but there are plenty others throughout history in business, warfare, and science.
No way to predict, sure. But if my memory serves me correctly, the entire theme of Taleb's Black Swan was about accounting for such events. For example, financial hedging; you won't know what event might manifest itself and cause a huge financial disruption (the cause), but you can anticipate "it" happening, in some form, and preparing for it by buying long shot hedges (accounting for the effect).
We can be fairly certain that something unexpected will happen in the future, but predicting what that something is or with what likelihood it will occur is still the domain of fortune-tellers.