Shadowblade can deploy using the Hidden special rule 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. (Clearly Shadowblade is not 'moving' between units, his disguise was simply so good that he was hiding just where he needed to be all along, waiting for the perfect time to strike!). 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.