The Black Bloc is an organized group of anarchist protesters who swear they are not organized, anarchists, or protesters. If pressed, they will tell you that "there is really no such thing as the black bloc."
"Black Bloc" is a tactic used by pussies that like to stir shit up for no other reason then to stir shit up. Which wouldn't be so bad if they didn't destroy the credibility of all the legitimate protesters. But then again, they're all just worthless hippie faggots, anyway.
Whether or not this is said in a "you don't talk about Fight Club" sort of way or with total sincerity remains unclear. The answer is probably somewhere in the middle but, regardless of the truth, it doesn't matter, anyway, since they're just emo hipster douchebags.
Their name is derived from the name created for youth in Germany who attacked banks after protests, not because they dress in black and wear black bandanas.