There already is a 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 windows-blue at this time, and agreeing with what @nhinkle 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 windows-8.1 should be made a synonym of 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 windows-8 and service-pack (like how there is only windows-7 and no SP1-specific tag).