Based on prior knowledge, the use of beta-blockers in the ICU (feature B_BLOK_S_n) can have an impact on the occurrence of chronic heart failure (target variable). To analyze the relationship between these two variables, we need to examine the distribution of B_BLOK_S_n values for both Yes and No classes of chronic heart failure.

Here is the dictionary with the required format:

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

Explanation: 

Since the feature is a categorical variable with only two possible categories ('no' and 'yes'), we can create the dictionary based on these categories. For the 'no' class of chronic heart failure, the only possible value for B_BLOK_S_n is 'no'. Similarly, for the 'yes' class of chronic heart failure, the only possible value for B_BLOK_S_n is 'yes'.

Note: If there are additional possible values for B_BLOK_S_n that are hard to predict but exist for either class, they should be included in the respective list in the dictionary. However, in this case, there is no such additional value.