Definition of Black Bloc in US English:

Black Bloc

noun

  • treated as singular or plural A radical anarchist group whose members dress all in black and conceal their identities when protesting.

    ‘as in Egypt and elsewhere, the Black Bloc in Brazil says it exists to protect other protesters from heavy-handed police tactics’
    as modifier ‘city officials said they expected as many as 10,000 protesters, including hardcore Black Bloc anarchists’