I think there's a chance that the "8.1" is an April Fool's joke, because there is evidence (based on rumors, mind you) that Windows Blue will be called Windows 9.
Why do I say that? (1) Because several job postings say that engineers are already testing Windows 9 on phones. If Windows 9 isn't due until November 2014, would they already be testing it now... or would they be testing Windows Blue? (2) Another rumor was that IE11 will be released with Windows 9. Okay, makes sense, but then why is it being integrated into Windows Blue? Would Microsoft really release a beta browser with an OS update (Windows Blue)? I doubt it. (3) Microsoft has said that they're going to speed up their update schedule. Well, is calling Windows Blue "Windows 8.1" really speeding anything up if Windows 9 won't be released until 2.5 years after Windows 8? (4) The kernel number for Windows Blue has increased to "6.2" (1/10th of a point), which is usually reserved for a new OS number, not a service pack. (5) Why give a code name to a service pack... or to just bump up the kernel number? I suspect that Windows Blue includes a lot more changes than are being leaked.
Again, this is all speculation on my part & I have no inside knowledge.