Groups of young anarchists (typically aged fifteen to twenty-five) who attend anti-globalization protests, such as QuebecCity or the BattleOfSeattle?. There is no one BlackBloc organization; you are a member of BlackBloc if you attend a protest in appropriate dress (see below). There is some overlap of members from one protest to the next and some people who attend only one protest.
BlackBloc members wear all black, hence the name. They are often masked with bandanas covering their faces from nose down. This gives them both anonymity and protection against tear gas.
BlackBloc is alternately seen as the heart and the scourge of the anti-globalization movement, mainly because they use (occasional) PropertyDamage and RetaliatoryViolence? as protest tactics. On the one hand, these tactics probably increase awareness of the protests in general; on the other hand, they may actually decrease the awareness of the other protestors. This issue is discussed intelligently in  and a way of resolving the problems raised there is presented in . For a discussion of this, see RaisingSocialCosts.
When mainstream newscasters and politicians refer to "a small group of hard-core anarchists, bent on destruction", they are probably referring to BlackBloc.
In my not so humble opinion, BlackBloc is definitely the scourge of globalization protestors. Their tactics, which, from their mouths, are specifically about provoking violent response from police forces, distort any other messages of the protests in general. It's not about RetaliatoryViolence?, it's about provoking violence so that you can justify retaliation. They definitely decrease awareness of the other protestors who easily out number BlackBloc. If other protestors try to stop BlackBloc, they will be attacked (ref. Genoa).
I liked the CanadianSecurityIntelligenceService? report on AntiGlobalization? (http://www.csis-scrs.gc.ca/eng/miscdocs/200008_e.html) but these days you can just read the CBC articles. American coverage seems less accurate.
Bit of a rant but I don't like people who cause violence and chaos, drag innocent people into violence and chaos, and then have the gall to try to come off as martyrs.
[somewhat DevilsAdvocate] It's hard to pick up a newspaper without seeing dramatic coverage of the violence, followed by a careful explanation that only a handful of protestors are violent, and the others have issues. Often, they go into extensive coverage of the issues. Would those issues be covered without the violence? Nobody can say for sure, but I doubt they'd reach as many people as they have.
More importantly, I believe the violence draws a lot of participants--even those who are opposed to its use. On a personal level, many people go to protests because they want to be where the action is. They won't throw bricks or even scale walls, but they want to be there if somebody else is going to.
On top of that, there's nothing like physical danger to unite a group of people quickly. I have seen coalitions 3 or 4 orders of magnitude smaller than the ones in QuebecCity or in GenoaItaly? fail due to personality conflict. I'm often stunned the protests don't collapse under their own weight, and I believe a lot of that is due to the immediate physical danger they feel. --DanKeshet
In QuebecCity, many of the protesters (esp. during the "boring parade") were part of a union. Others were united because the left is generally tolerant by definition. However, after the violence started, many people rallied together. A common enemy makes many impromptu friendships. I don't like the people in the BlackBloc normally, but for a while I was on their side. Worse, for a while I had their agenda (although I remained an observer). You'll see a lot of so-called pacifists lose their philosophies when the bricks start flying. Some have accused the police of instigating this themselves at various such protests by planting provocateurs in the crowd. I doubt that happened at QuebecCity, but the police certainly do become an all-consuming devilhead. -- SunirShah