In fact, there are many black bloc anarchists who don't wear masks during blocs. Anarchists wear masks for many reasons. The main one is the recognition that the police videotape activists and keep files on them. Masks promote anonymity and egalitarianism. They also protect the identities of those who want to engage in illegal acts and escape to fight another day.
Tactics vary from black bloc to black bloc.
Some common ones include unarresting and arm-linking . Unarresting is where the bloc frees people who don't want to be arrested. This usually works if you outnumber the cops. It also works because most cops are shocked that activists would try and free someone. Arm linking, or locking, helps a bloc maintain cohesion and makes it harder for the police to disperse members.