A blacklist is a list of message recipients that decided to opt-out of receiving further SMS messages. Blacklists are mostly used for needs of promotional campaigns where companies give an option to the end customer so he or she does not receive messages irrelevant to them. In some countries having an opt-out inside every SMS message is obligatory by law.
Also, chat apps like Viber have the opt-out option by default for all messages coming to their users. Besides the blacklists, owned by every company for their messaging activities, there is also a DND or Do Not Disturb list.
Those lists are used by the Mobile Network Operators for the protection of their users. If one of their subscribers decides to be on the Do Not Disturb list, they will not receive promotional messages, no matter if they’re subscribed to a newsletter or not. This is a method used by MNOs to fight against churn rate and customer spamming.