Declaring blockers doesn't go on the stack.
The following is from the Rules Q&A Main Rules FAQ:
Q: If you're playing with more than 1 person (3 people) and player A attacks player B, can player C block this?
Q: If all the creatures blocking a particular creature are somehow removed, does that make the creature unblocked?
A: No. Once a creature becomes blocked, it stays blocked for the rest of combat, no matter what happens to the creatures blocking it.
Q: If more than one creature blocks an attacking creature, how is the damage distributed?
A: The attacker chooses the order of the blockers. A blocking creature cannot be assigned damage until all preceding blockers have been assigned lethal damage. In the case of deathtouch, 1 damage is lethal. A blocker can be assigned more damage than its toughness can handle if the attacker does not wish to assign damage to the second blocker; however, if the attacking creature has trample, all blockers must be assigned lethal damage before any damage can be assigned to the blocking player.
A: No. Once blockers have been declared, that's it. It doesn't matter whether the creature has some sort of ability that would prevent it from being blocked if you went through declaring blockers again--it's blocked now, and that's all that matters..
Q: When is my last chance to boost my creatures/shrink my opponent's creatures to make them deal more/less damage in combat?
A: If you want to change the size of a creature so that it deals more or less damage, you have to do it before that creature's combat damage is assigned. Once a creature assigns damage, that damage is dealt as originally assigned, no matter whether or not the creature has changed in the interim.
In most cases, that means you have to pump/shrink the creature during the Declare Blockers step. (If first or double strikers are involved in combat, you can wait until the first strike combat damage step to pump/shrink non-first-strikers.)
Q: My opponent attacks or blocks with a particular creature--can I then do something before he attacks or blocks with his other creatures?
A: No. All attackers are declared at once, and all blockers are declared at once--there is never a time "between" two creatures attacking or blocking during the same turn.
Q: My opponent wants to block one of my attacking creatures with a creature that is tapped. Can they do this?
A: No. Tapped creatures cannot be assigned as blockers.