There already is a tag: [tag:Windows-8.1] (16 questions currently).  As of posting this, the tag summary reads:

 > Windows 8.1, code name "Blue", is an update to Windows 8 by Microsoft, scheduled for release in late 2013.

There is no such tag called [tag:windows-blue] at this time, and agreeing with what [@nhinkle][1] stated below, I don't think it's necessary unless people start using the tag (Windows 8.1 is the more official name).

As of yet, I am not sure if [tag:windows-8.1] *should* be made a synonym of [tag:windows-8], in the cases where issues do happen to be specific after upgrading to the latest version.  For those particular questions however, if the two were made synonyms, I think it would be just as clear to tag the question with both [tag:windows-8] and [tag:service-pack] (like how there is only [tag:windows-7] and no SP1-specific tag).


  [1]: http://meta.superuser.com/users/20088/nhinkle