Can you sacrifice a creature that is already being declared as a blocker?

Declaring blockers happens during the Declare Blockers Step (CR 509). Once a creature is legally declared as a blocker, it is 'blocking' — but that status does not protect it from being sacrificed by spell or ability effects at instant speed. Sacrificing is a cost or effect, not a game action restricted to sorcery timing.

After blockers are declared, both players receive priority (CR 509.4). At that point, either player may cast instants or activate abilities — including those that require sacrificing a creature. The declared blocker can be sacrificed as part of such a cost or effect.

If the blocking creature is sacrificed before combat damage (CR 510), it leaves the battlefield and is no longer a legal blocking creature. However, the attacker it was blocking does not automatically become unblocked — a creature that was blocked remains blocked even if the blocking creature leaves the battlefield (CR 509.1h, 702.2c does not apply here; see CR 509.1h explicitly). So the attacker still will not deal combat damage to the defending player unless it has trample.

Concrete example: You attack with a 3/3. Your opponent blocks with their 2/2. After blockers are declared, your opponent activates an ability that says 'Sacrifice a creature: gain 3 life.' They sacrifice the 2/2 blocker. The 3/3 is still considered blocked and deals no combat damage to your opponent (unless it has trample), but the 2/2 is gone and deals no combat damage either.

Bottom line: sacrificing a declared blocker is always legal when you have priority and an applicable effect — the key consequence is that the attacker remains 'blocked' despite the blocker's removal.