Blacklist is a list, e.g. a list of people, email addresses, suppliers etc., which tells who or what should be concerned by some kind of limitation. It is a list of items who are restricted from doing something, eliminated from some kind of processing or activity, and so on. The opposite is called whitelist, a list of those who are not concerned by a limitation - who are allowed to do something, concerned by something, should be involved in something, and so on.
When do we make use of blacklist?
For example, you can create a blacklist of senders in your email client in order to block their incoming (junk) e-mails. Moreover, each email service provider has its own blacklist, which blocks many unsolicited messages before they even reach your mailbox. Another example of blacklist is the list of forbidden or illegal website or IP addresses installed on the router or firewall. This will stop the users from visiting the listed websites and from connecting to the listed IP addresses.
In principle, blacklist can be used whenever we need to exclude or block some items from a particular list.