Can you cast spells during an opponent's declare blockers step?

After blockers are declared, both players receive priority in turn order before the game moves to the combat damage step. Any player who has priority may cast instants or spells with flash, activate abilities, and take other legal actions. This is explicitly supported by CR 509.4, which states that after blockers are declared, the active player receives priority and the combat damage step does not begin until all players pass in succession.

This makes the declare blockers step a very common window for combat tricks. You can cast a spell that gives your creature a boost, kills a blocking creature, grants trample or first strike, or otherwise changes the combat outcome — all before damage is assigned.

It's important to note that you cannot cast sorcery-speed spells here unless you have a special permission (such as a card that says "cast this as though it had flash"). Only instants and abilities are normally available at this point (CR 307.1, 116.2a).

Example: Your opponent attacks with a 3/3 and you block with your 2/2. After blockers are declared, you cast Giant Growth (an instant) targeting your 2/2, making it a 5/5. Now your creature survives and destroys the attacker in the damage step — a legal play made during the declare blockers step.

Remember that your opponent also has priority and can respond to your spell before it resolves, so timing and the stack still matter normally (CR 117.1, 509.4).