The role of the black bloc, as I remember my friends telling me, was to mar otherwise peaceful protests with looting and violence. So it sounds pretty similar to what you describe in Oakland.

I remember Dave’s and my friend Steve getting up in from of our environmental studies class and talking about his trip to Seattle. He really wanted to emphasize how peaceful everyone was except for the few black bloc “anarchists,” whom he portrayed as ruining things for everyone else since they got the most media attention.