Every time windows are smashed and clashes erupt with police at Occupy protests in Oakland and San Francisco, one group is in the thick of it: masked, black-clad anarchists known as the Black Bloc. Police say they are pure trouble and point to conflicts at last week's May Day rallies as the most recent example. Most pacifist protesters wish they would go away. Hard-core Occupiers say they like having them around to diversify their movement's tactics. Exactly who makes up the Black Bloc is, by design, a bit of a mystery. Adherents rarely reveal their identity and say they aren't...