Blacklist

By blacklisting what is meant is that by policy everything is allowed with individual policies in place to deny someone. In IP firewalling one can use blacklisting to deny one or more IP's. In OpenBSD's spamd a large list of Chinese and Korean IP blocks have been installed to blacklist these IP's without allowing them to be greylisted. In TCP Wrappers the blacklist is in /etc/hosts.deny. On a mailing list where new posters pop up to ask intelligent questions one may blacklist an email address that is deemed annoying but let the others through. This is called a killfile or procmailing someone.

In IP firewalling it rarely makes sense to blacklist since threats can come from everywhere, it's much easier to whitelist an organization and deny the rest.