Blacklisting a subscriber means that they will not receive any future campaigns. This is a permanent action: it cannot be undone or reversed (see Remove Subscribers from Blacklist).
Subscribers can be manually blacklisted by you, manually blacklisted by the subscriber themselves (by unsubscribing), or automatically blacklisted by phpList if campaigns sent to them bounce consecutively many times.
The blacklist is maintained free of charge: if you have a pay-per subscriber plan, subscribers on the blacklist are not counted towards your total.
See also ‘How to delete subscribers‘ in the phpList manual.
Remove Subscribers from Blacklist