This tag is used in requests for adding one or more tags to the blacklist, preventing them from ever being recreated.
Blacklisting a tag removes all occurrences of it from the target site and prevents anyone from ever using that tag again.
Tags must be "quite bad" to be blacklisted. Generally, these are tags that could never convey any useful information or are actively harmful to the site. Because of the potential consequences of getting something wrong, only the dev team is able to add tags to a site's blacklist.
See The Death of Meta Tags for a discussion of what makes a tag good or bad.
Reference: import excerpt and wiki from the main Meta Stack Exchange site.