Question:
If a creature is blocking another creature and the attacker uses an instant to do damage to the blocking creature, does the damage stay through the damage phase or does the creature regain its health?
Thanks
Damage marked on a creature will remain on that creature, effectively, "until end of turn", whether that damage was dealt as the result of an instant spell or otherwise (C.R. 119.6, 514.2). And at the moment of assigning combat damage, damage already marked on a given creature is taken into account when determining whether lethal damage is assigned to it (C.R. 510.1c-d).
If a creature is blocking another creature and the attacker uses an instant to do damage to the blocking creature, does the damage stay through the damage phase or does the creature regain its health?
Thanks
EDIT (May 14, 2019): Correctness edit.