The block button works more or less the same as it did in prior versions. You should click and hold it down so it lights up (or use the page down key on the keyboard). When you are blocking an incoming attack will result in your HUD becoming charged. The only exceptions to this is if your stamina is too low. Charging uses some stamina points so if stamina is too low it will not charge. It will however still stop incoming damage. Another point all players should keep in mind when blocking that it is directional. ie. It will only block an attack coming from the front. If an attack comes in from behind you or too far to the side it will not have affect.
The only thing that is a little different in 4.1 from prior versions is that when you block and charge, the charge will now last for about 4 seconds even if you take a few hits before delivering the counter attack. Prior versions of the hud lost all charge when a hit was taken making a block-counter attack pretty hard to actually do.