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

Based on the feature "B_BLOK_S_n" and the task of determining if the patient's myocardial infarction complications data shows chronic heart failure, the possible values of the feature for each target class are:

- For the target class "no", the only possible value of the feature "B_BLOK_S_n" is "no".
- For the target class "yes", the only possible value of the feature "B_BLOK_S_n" is "yes".

Note: It is important to mention that if there are any other values of the feature "B_BLOK_S_n" that are not mentioned here, they will not be included in the dictionary.