This is Smogon's description of damage reducing berries
Halves damage taken from a supereffective (enter type name) attack. Single use.
1) So, what will happen if the berry is held by a Pokemon who does not take super effective damage from the berry's type. For example, what will happen if Mew (A Psychic Type) holds a Chople Berry (Reducing Fighting type damage). Will it reduce damage if Mew is hit by a Fighting type or not?
2) This is specifically regarding the Colbur Berry (The Dark type one). What will happen if Mew holding a Colbur berry is hit by Knock Off? Will the damage be reduced or not?