Shadowblade can deploy using Hidden like an Assassin. If he does so, he can change which unit he is hiding in at the start of any Movement or Close Combat phase – keep a note of where Shadowblade is each time you change your mind. If an opponent has an ability that forces you to state that there are 'hidden' models within a unit, you only need to say that Shadowblade is hiding within a unit, but not which unit he is currently in.