The use of aircraft as weapons on 9/11 could be a Black Swan as defined by NT but the market itself was already in a downtrend. From that point of view, the subsequent drop in the Dow etc. was a downtrend continuation, and nobody should have been surprised that price went further down. That particular event and type of event was obviously a surprise, but it is part and parcel of downtrends that some event will occur that has a further negative effect on price. The reverse for uptrends as its also possible to have a positive Black Swan
