Ravaging Blaze targets only the creature. It doesn’t target any player, even if the spell mastery ability applies.

If the creature becomes an illegal target by the time Ravaging Blaze tries to resolve, Ravaging Blaze will be countered and none of its effects will happen. No damage will be dealt to the creature or its controller.

Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies. The spell itself won’t count because it’s still on the stack as you make this check.