As a general rule in human history unpredictable events seem negative when they happen, the greater the unpredictability the greater the negativeness seems to people when it happens because unpredictability tends to destabilize systems set up to deal with predictable things, this however does not mean that the later consequences will remain negative for ever and ever.

All that said however, probably the greatest black swan in terms of unpredictability in human history was that there were two freaking big continents in between Europe and Asia when you try to circumnavigate the globe.

on edit: probably should be "the greater the unpredictability and the more things the unpredictable event connects to the greater the negativeness seems to be", because obviously the platypus while completely unexpected affected basically nothing at all.