They might mean a swan that is black on one side but white on the other, i.e. gives the appearance of being an extremely rare or a very common event, while in fact being the opposite.

Surely that's even more rare than a black swan?

Lol

Ironically, a black swan isn't really at all - just unknown. Which really is Taleb's point: black swans aren't as rare as they are priced to be.