In adopting the black outfit the Black Bloc has made themselves easier to single out.
What about doing something a little more sophisticated like, say, everyone wears jeans and sneakers and rolls a die to choose :
Baseball cap, bandana, t-shirt color.
Limit the garb to a small set of colors. red white blue green yellow black
(1) Black is the "traditional" color of the Anarchists. (2) Black has societal connotations that none of those do. (3) Black (or any really dark green/brown/blue) has other properties like making the wearer look a little thinner, causeing some slight shifts in color perception etc. It is also harder to tell different tints and shades of black apart, while if you said "Where a red shirt", there will be 37 different shades of red. (3) The point is not to be anonymous from the crowd around you, but to be anonymous within the group. These people aren't worried about post-analysis of the images, they don't think that far ahead. If they did, they wouldn't be that kind of anarchist (which I am assuming is basically some variant on anarcho-syndicalism).
Want to intimidate? An entire crowd chanting some 3-word slogan in unison is probably as scary as the black outfit.
Want to *really* intimidate? Hack the police's CnC network.
Let's see observers identify the presence or location of the group, assign membership to a particular person or track a subgroup through a crowd. Who's going to get three colors correct during a pursuit when those colors can be lost or changed on the fly?
The goal is not to be part of the crowd during a protest, but to be able to accomplish specific actions with a reduced likelyhood of arrest. Given adequite warning, and the proper training it wouldn't be hard for police to cut this group out of the herd and arrest all of them. As another poster mentioned, RICO could probably be brought to bear on them.
A single color is not so anonymous.
-- http://www.apa.org/journals/psp/psp7761121.html It is one of the essential features of such incompetence that the person so afflicted is incapable of knowing that he is incompetent. To have such knowledge would already be to remedy a good portion of the offense.