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.
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.