Blocking is a well-studied learning phenomenon in which previous learning inhibits subsequent learning about novel cues. Existing models provide different explanations for blocking and predict different beliefs about novel cues early in the second phase of blocking. Two experiments examined learners’ beliefs when first encountering novel cues. The results suggest that the introduction of the novel cue in the second phase of a blocking paradigm adds uncertainty and that learners entertain the possibility that novel cues are preventative. A novel computational account is proposed to explain people’s beliefs, because existing models cannot fully account for these findings.