1) They cannot be "accurately" predicted. I predict one day an alien space ship will land on the white house lawn. Two hundred years from now, it actually happens. Does that make it a black swan? Yes. Sure, I predicted it, but there is no way I can accurately predict exactly when aliens will land on the white house lawn. What good is a prediction if there is not an accuracy component to it? That's like saying one year; the Cubs will win the World Series. Which year! Anybody can throw shit against the wall; some of it is bound to stick.
2) After the event happens, it has to appear to be obvious in hindsight. Once aliens make contact with us, everyone will say, well of course, it was bound to happen sooner or later.
3) And the 3rd one, the event has to have significant importance. Aliens landing and making contact with us would certainly change the way we see the world and we would never be the same.
Some events which are black swans are 9/11, Google, the first World War, the computer, etc. As you can see, black swans can be both positive AND negative. None of these events were accurately predicted. Sure, you can make a very broad blanket statement; one day there will be a nuclear holocaust. Then when it happens, you will say you predicted it. But the fact of the matter is, you will NEVER predict when that will happen. Saying that it will happen in the next 1000 years is not a prediction. What were some other possible black swans? The devaluation of the Deutsche Mark in 1920, the AIDS virus, the assassination of Jack Kennedy, etc.