responding to blockers with protection mtg

[Smash]

can you respond to declaring blockers by granting protection of the blocking creature's color to the attacker?

[Arix]

You can't respond to "declaring blockers". As soon as blockers are declared, they start blocking whatever their controller wants. No chance to respond.

However, once all blockers have been declared and started blocking, you can play instants or activated abilities that grant protection. This won't stop the creatures from blocking, but it will prevent the damage they'd do to the attacker.