I'm thinking that it might help new comers who don't have enough reputation more options to arrive at a dummy tag, instead of giving up posting the question because the tag for the series they want to ask is not available.
I think what you're trying to solve should be handled a bit differently. If a user doesn't know what to type into tags list, they may type "anime" or "manga" and then a popup would appear explaining how to use tags if one of those blacklisted words is detected.
Currently it's this:
That message is not very helpful at all.
To make it more user-friendly, some solutions would have to be added, like "Add the name of the anime/manga you're asking about". The message itself has to be enough for a new user to know what to do, without resorting to links to relevant meta posts, which are very long and you know it's not a nice solution. But we could think of a good way to solve this.