I should also add that you shouldn't become dependant on blocking. Sure, everything has a time and a place, but if you become dependant on one set thing, it'll work against you.

With most blocking, you defend against one thing only to expose something else.

One problem I see with alotta amateur fighters today is that they do things the same way over and over. Now if you're the opponent and you see that he keeps blocking when you throw a certain punch, you could try to set him up. So you make him think you're going here-- then boom you hit him somewhere he doesn't expect it.