There already is a 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 windows-blue at this time, although I would support it becoming a synonym of windows-8.1. As of yet, I am not sure if windows-8.1 should be made a synonym of windows-8