Potiphar's Wife will not capture a male Hero if she is discarded to Failed Objective.
PW must defeat the Hero to capture them. Although the block is successful because FO protects the Souls from rescue, Potipher's Wife is the one who is defeated. The Hero wins the battle, it just happens to be a battle challenge instead of rescue attempt.
Potiphar’s Wife
Type: Evil Char. • Brigade: Gold • Ability: 2 / 2 • Class: None • Special Ability: When character defeats a male Hero in the Field of Battle, the Hero is taken prisoner and placed in your opponent's Land of Bondage and treated as a Lost Soul. • Play As: All male characters defeated (by a win or stalemate) by Potiphar’s Wife in battle are taken prisoner and placed in your opponent's Land of Bondage and treated as a Lost Soul.
Failed Objective
Type: Evil Enh. • Brigade: Gold • Ability: None • Class: None • Special Ability: Discard all Evil Characters in battle. All Lost Souls are protected from being rescued by Heroes this turn.