Official FAQ wrote:
(1.29) Victory Display
The victory display is a game area where victory points are tracked. Cards in the victory display are considered to be out of play, but are not considered to be a part of the encounter discard pile. Cards in the victory display are not considered “removed from game,” and some card effects may still interact with them.
The trigger for this Response happens the moment you have declared an attack with the Hero that has Black Arrow attached. If you want to use it, move it to the Victory Display and receive the benefit of +5 Attack for this attack and 1 Victory Point at the end of the game. So it's a one-shot baddie killer.