What is a blocked tag?
A blocked tag (formerly blacklisted tag) is a tag that can no longer be used in a specific Stack Exchange community. Blocked tags are automatically prevented from being used or created on a site.
What are the reasons for blocking a tag?
A tag may be marked as blocked 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.
Generally, tags are only blocked if they have been recreated and removed multiple times despite such consensus. Some communities may also block a tag if a significant number of questions are using it, to prevent new questions from using the tag while the community continues to manually remove the tag from existing posts which use it.
Examples of blocked tags include, but are not limited to:
Who can mark tags as blocked?
Tags can only be added to the block list by Stack Exchange employees. However, you can request a tag be blocked by posting with tag-block-request on the relevant per-site meta. (See How should we make requests to block tags? for more information).
Why am I being prompted to remove a tag from a question I'm editing?
It's possible to block tags that already currently exist, and blocking a tag by itself does not automatically remove the tag from questions previously using it. However, if you edit a question that currently uses a blocked tag, you will be prompted to remove it before you can submit your edit.
See also: Why don't we block all tags that we remove?