black blocs are typically spun as part of an underground army
like Fight Club
's Project Mayhem
, black bloc is actually a strategy
adopted by groups rather than being a group.
Certain groups adopt black bloc (or green bloc, or red bloc...) in order to preserve anonymity. This makes it easier for people committing aggressive or illegal actions to disappear into the crowd, as well as protecting the identites of the participants.
Black bloc is most effective at large demonstrations, where there are a lot of people, and especially if significant numbers of those present are dedicated to nonviolent tactics.
Ironically, black bloc is a very frightening tactic for most police officers, who are engaging in a similar tactic via their uniforms - police uniforms, or blue bloc, also serve to preserve anonymity and allow cops to take illegal and aggressive actions without fear of reprisal.