It is a tactic used by anarchists and egalitarians in opposition to government-backed corporate control of peoples lives and the economy that we all exist within. During the week of the Democratic Party National Convention in LA many persons masked up and wore black clothing to join the marches as a large and visible contingent of invisibles. I wonder what that message would have been. We just were there. And maybe that confused some people.
Black masks provide anonymity and safety for individuals yet visibility as a group.
And they know it. If you've never noticed someone and then they put on a black mask and you can suddenly see them, that isn't necessarily a message, but something has changed. Every organism, life itself, will fight to stay alive. It is healthy to be willing to protect yourself to stay alive. Our presence represented a growing healthiness of the movements to end corporate dominance.