What is a blacklisted tag?
A blacklisted tag is a tag that can no longer be used in a specific Stack Exchange community. Blacklisted tags are automatically blocked from being used or created on a site.
What are the reasons for blacklisting a tag?
A tag may be blacklisted for a number of reasons. It may be a redundant tag, such as a game tag on Arqade or a programming tag on Stack Overflow. It may be a meta tag describing the character rather than the topic of a question. In general, there is usually a shared consensus that these tags add little to no value to a question. The blacklist is usually only employed in cases where a tag has been recreated multiple times despite such consensus.
Examples of blacklisted tags include, but are not limited to:
Who can blacklist tags?
Tags can only be blacklisted by Stack Exchange employees. However, you can request a tag be blacklisted by posting with tag-blacklist-request on the relevant per-site meta. (See "How should we make tag blacklist requests?" for more information).
Why am I being prompted to remove a tag from a question I'm editing?
Blacklisting, by itself, does not automatically remove the tag from questions using it before it was blacklisted. However, if you edit a question that currently uses a blacklisted tag, you will be prompted to remove it before you can submit your edit.
See also: Why do we burninate instead of blacklisting?