As of right now, the windows-8.1 tag has 16 questions currently, and the tag summary reads:
Windows 8.1, code name "Blue", is an update to Windows 8, scheduled for release in late 2013.
Regarding the two questions posed above:
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).
If we look at the existing tags for some recent Windows operating systems, and contrast this with the kernel numbers for all service packs for that OS (excluding Windows 8, more on that next), are windows-xp (5.1.XXXX), windows-vista (6.0.XXXX), windows-7 (6.1.XXXX), and finally, windows-8 (6.2.XXXX).
After seeing @Ramhound's comments below, I decided to see why they didn't call it a service pack (e.g. Windows 8 SP1), and sure enough, it seems as though the public preview of Win8.1 has a kernel version of6.3.XXXX (inconsistent with service packs).
For this reason, I would argue the tags should remain separate, unless the version numbers are further changed before the official release of WIndows 8.1.