If the language keys don't exist in your CB language plugin you will need to add them with language overrides. Those are as follows.
Key: BLOCK_MESSAGES_FROM_USER
String:
Code:
Block Messages from <span class="text-info">[user]</span>
Key: UNBLOCK_MESSAGES_FROM_USER
String:
Code:
Unblock Messages from <span class="text-info">[user]</span>
They are not always in the actual language.php file especially for defaults which load from a plugins language folder. Be sure to use the language finder on the language overrides page instead of checking files directly. If overriding for French you must put your override in the French language plugin or they will not load (language overrides do not fallback to default). The keys will always exist in the default language.
Language plugins sometimes are missing language keys as it's an entirely manual process to get Transifex updated. We're reviewing self-hosted solutions so we can finally automate this entire process.