I liken the concept of a black swan with the character referred to as, "The Mule" in Isaac Asimov's Foundation Series. Hari Seldon invents Pschyo-History, a technique to predict the future, not of individuals, but of society as a whole. The Galactic Empire is on a deathly spiral and faces impending doom. Seldon uses his techniques to put into motion a plan to rebirth the Galatic Empire in a significantly shorter period of time than would naturally occur without his intervention. This secret plan depended upon the accuracy of Seldon's psycho0history.
That plan faces an unexpected problem with the advent of The Mule, a mutant character who uses his super-human abilities to sense and install fear in others. He uses his power to take control of the entire empire.
I'll leave the rest of the story up to you, but the point is The Mule was unplanned for and his unnatural abilities resulted in potentially massive shifts in history with his erratic and extensive behavior - much like that of a blank swan event.
