Microsoft removed the code that allows 'blur behind' in glass windows. The API call itself is still there and available, but it doesn't work. They did the same with the Windows Start Menu code, so nobody else can use it.
I think it's the first time Microsoft ever removed functionality from Windows. Way to go, Microsoft! Not.
Since the blur behind function relies heavily on hardware shaders and is probably also tied to low level window code, adding it back will not be easy. This said, if anyone might pull it off, it's Neil.