Can you tap a land for mana during the declare blockers step?

Mana abilities are special: they don't use the stack and can be activated any time you have priority, or any time a rule or effect asks you to pay a cost — even during combat (CR 605.3a). The declare blockers step is no exception.

After blockers are declared, both players receive priority in the declare blockers step (CR 509.4). While you have priority, you may activate mana abilities, including tapping lands for mana. You do not need a specific 'window' for mana abilities the way you do for spells or non-mana activated abilities.

The most common practical reason to do this is to have mana ready to cast an instant or activate an ability — for example, to give a blocking creature a boost after blockers are declared but before combat damage is assigned.

Concrete example: Your opponent attacks with a 3/3. You declare your 2/2 as a blocker. After the declare blockers step begins and you receive priority, you tap a Forest for green mana, then cast Giant Growth targeting your 2/2, making it a 5/5. It now survives combat and destroys the attacker.

Key rules: CR 509.4 (priority in declare blockers step), CR 605.3a (mana abilities can be activated any time you have priority), and CR 117.1 (when players receive priority).