The term 'black swan', invented and popularised by Nassim Taleb, clearly fills a conceptual niche, given its popularity in the futures community. Analytical failures can be broadly divided into two categories: failures of imagination, and critical thinking errors. As used by Taleb, a 'black swan' is a failure of imagination in which, instead of a high-impact hypothesis or outcome being appraised and discounted, it is simply not considered at all. Taleb's story is that until black swans were actually discovered, it was simply assumed that they did not exist.
"Before the discovery of Australia, people in the Old World were convinced that all swans were white, an unassailable belief as it seemed completely confirmed by empirical evidence." - The Black Swan, Nassim Nicholas Taleb
There are at least two reasons why the term 'black swan' might be a less than ideal term to use for this concept. First, and perhaps most boringly, there is plenty of evidence that the concept of a black swan was considered well before the discovery of Australia. Perhaps more conceptually problematic, though, is that the notion of a black swan can be generated fairly easily and algorithmically by simply combining known colours with known animals. This makes it a less interesting sort of failure than the failure to consider objects with characteristics that are entirely unlike anything previously encountered.
Necessarily, it is not easy to think of examples. Candidates might include the arrival of settlers to the New World, quasars, or Facebook. In Flann O'Brien's remarkable novel 'The Third Policeman', the narrator hears a story about a box which contains a card that is of a completely unimaginable colour, one which sends anyone who sees it insane: "I thought it was a poor subject for conversation, this new colour. Apparently its newness was new enough to blast a man's brain to imbecility by the surprise of it. That was enough to know and quite sufficient to be required to believe. I thought it was an unlikely story but not for gold or diamonds would I open that box in the bedroom and look into it."