```json
{
    "no": ["no"],
    "yes": ["yes"]
}
```

Based on the given feature description, the categorical feature `B_BLOK_S_n` represents the use of beta-blockers in the ICU. To determine if this patient shows chronic heart failure, we need to analyze the relationship between this feature and the target variable. 

Since the feature has only two categories, "no" and "yes", we can directly map these categories to the target variable. If the value of `B_BLOK_S_n` is "no", it suggests that the patient did not use beta-blockers in the ICU. On the other hand, if the value is "yes", it indicates that the patient did use beta-blockers in the ICU.

As there is no additional information provided that suggests any other possible values for `B_BLOK_S_n`, we can conclude that these two categories are the only relevant values for this feature in relation to the task. Therefore, the dictionary shows the possible values of `B_BLOK_S_n` for each target class, "no" and "yes".