Can you tap a land for mana after blockers are declared but before damage?

After blockers are declared (the Declare Blockers Step), both players receive priority in turn order before the game moves to the Combat Damage Step. Any time a player has priority, they may activate mana abilities, which includes tapping lands for mana. This is explicitly supported by CR 509.5, which states that after blockers are declared and any triggered abilities resolve, the active player gets priority.

Tapping a land for mana is a special type of activated ability called a mana ability (CR 605.1). Mana abilities don't use the stack and can be activated whenever you could activate an instant-speed ability — including during combat when you have priority (CR 605.3a).

This matters most when you want to cast an instant or activate an ability in response to the combat situation — for example, pumping a creature with a mana-costing instant after blockers are assigned but before damage is dealt. You tap your land to generate that mana, then spend it on the spell or ability.

Example: Your 2/2 attacks and your opponent blocks with a 3/3. After blockers are declared, you tap a Forest for green mana and cast Giant Growth on your attacker, making it a 5/5. Combat damage is then assigned with your now 5/5 creature, which destroys the 3/3 and survives.

There is no restriction on tapping lands for mana between the Declare Blockers Step and the Combat Damage Step, so long as you have priority. Just remember that mana empties from your mana pool at the end of each step (CR 500.4), so spend it before the step ends.