What happens when I sacrifice a blocking creature?
There was a fairly long period of time ending with Magic 2010 when you could put damage on the stack, and then play instants and abilities before the damage resolved.  Those days are over!

Nowadays, you declare blockers, and then both players have a chance to play spells and abilities.  Once everyone is finished doing this, we move onto the combat damage phase, and only then do creatures even start thinking about dealing damage to each other.  If you sacrificed your creature after declaring it as a blocker, then it never gets an opportunity to deal damage, I'm afraid!

One corner case: if your creature has First Strike and also a sacrifice ability, you can have it deal its first strike damage and then sacrifice it in the window of opportunity both players have to play spells and effects before normal combat damage.  Try this page for a handy breakdown of when players have a chance to do things in the course of a Magic game.

Also have a look at Magic 2010 Rule Change, explaining some of the latest changes made to the game rules, particularly section 5, about combat damage not using the stack.  Much more readable than the Comprehensive Rulebook, I think you'll find...