You can block, and then still in the declare blockers step, sacrifice it. The attacker will remain blocked and won’t assign combat damage to the player or planeswalker it’s attacking unless it has trample or a similar ability. The creature you sacrificed won’t be there to deal combat damage either though. You can’t wait until damage is dealt, because your creature will die from it before you get the occasion to activate its ability.
As an aside, note that damage does not reduce toughness. It gets marked on the creature, and the creature is destroyed if the damage marked this turn equals or exceeds its toughness.
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
If a creature's toughness is 0, it is immediately put into the graveyard as a state based effect, without either player gaining priority before its gone.
Note, though, that you are a bit confused about a few things. Nothing about combat will normally make a creature's toughness 0 unless you are using an effect that gives -X/-X, like Wither or Dismember. Combat damage is marked on a creature but does not change its toughness.
To fully answer your question, no, you cannot sacrifice a creature after it has been dealt lethal combat damage. You can sacrifice it right before combat damage is dealt, but it will not deal combat damage to the creature it was blocking, nor will the blocked creature deal damage unless it has trample and vice versa.
Thanks!
As an aside, note that damage does not reduce toughness. It gets marked on the creature, and the creature is destroyed if the damage marked this turn equals or exceeds its toughness.
Note, though, that you are a bit confused about a few things. Nothing about combat will normally make a creature's toughness 0 unless you are using an effect that gives -X/-X, like Wither or Dismember. Combat damage is marked on a creature but does not change its toughness.
To fully answer your question, no, you cannot sacrifice a creature after it has been dealt lethal combat damage. You can sacrifice it right before combat damage is dealt, but it will not deal combat damage to the creature it was blocking, nor will the blocked creature deal damage unless it has trample and vice versa.