There already is a tag: [tag:Windows-8.1].  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, although I would support it becoming a synonym of [tag:windows-8.1].  As of yet, I am not sure if [tag:windows-8.1] should be made a synonym of [tag:windows-8]