Can you sacrifice a creature to pay a cost after blockers are declared but before combat damage?

After blockers are declared in the declare blockers step, both players receive priority and may activate abilities or cast spells before the combat damage step begins (CR 509.5). This means you can absolutely sacrifice a creature — either an attacker, a blocker, or an uninvolved creature — to pay the cost of an activated ability during this window.

Activated abilities follow the format 'Cost: Effect' (CR 602.1). If that cost includes sacrificing a creature, you simply pay it when you activate the ability. Sacrificing is a special action used as a cost — it does not use the stack itself, but the ability it pays for does (CR 602.2a).

It's important to note that if you sacrifice a blocking creature, the attacker it was blocking does not automatically become unblocked. Once a creature is declared as a blocker, the attacking creature remains blocked for the rest of combat even if the blocker leaves the battlefield (CR 509.1g). The attacker will simply deal its damage to nothing (unless it has trample).

Concrete example: Your opponent attacks with a 3/3. You block with your 2/2. Before damage, you activate Phyrexian Altar by sacrificing your 2/2 blocker to add one mana of any color. The 2/2 is gone, but the 3/3 is still considered blocked and will not deal combat damage to your opponent unless it has trample.

Similarly, you could sacrifice an attacking creature you control to pay a cost — that creature simply deals no combat damage, since it's no longer on the battlefield when the combat damage step is reached (CR 510.1).