Shadowblade can deploy using the Hidden special rule like a Khainite 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 the 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 within.