Can you cast an instant spell during the declare blockers step?

The declare blockers step is one of the four steps of the combat phase, and like most steps, it includes a round of priority after the active game actions are completed. Once the defending player has declared blockers (or chosen not to block), both players receive priority in turn order and may cast instant spells or activate abilities. (CR 509.4)

Priority works the same way here as anywhere else: the active player (the attacker) receives priority first, then the defending player, and so on back and forth until both players pass priority in succession with the stack empty — at which point the game moves to the combat damage step. (CR 117.3)

This is an extremely common and important window. For example, combat tricks like pump spells, removal spells that destroy a blocker or attacker, and spells that grant first strike are all typically cast right here, after seeing how blockers have been assigned but before damage is dealt.

Concrete example: Your opponent attacks with a 3/3 and you declare your 2/2 as a blocker. Before damage, you cast Giant Growth targeting your 2/2, making it a 5/5. This is cast during the declare blockers step, after blockers are declared. Your now-5/5 will deal 5 damage to the 3/3 when combat damage is assigned in the next step.

Note that spells cast in this window resolve before combat damage is dealt. If you want to affect the combat result, the declare blockers step is typically your last clean opportunity to do so before damage is assigned (unless a spell grants first strike or double strike, which adds an additional damage step).