I mean the blur under all the ui parts : title bar (caption), taskbar, windows sides and bottom, contextual menus....
For example, let's say that you design a skin with rounded corners for the windows and enable the blur in per pixel frames, then you'll find that the windows have squared corners blurred under your lovely round corners. If you add shadows (by yourself or using windowblinds option), you'll have a really unpleasant non pro skin : shadow + blur layer (with squared corners and a blue ghost for the inactive state) + your skin showing emptyness between it's rounded corners and the edge of the blurred layer.
A picture is always worth a thousand words, here's a little example....
Background + shadow + blur layer with squared corners (per pixel settings option) + skin with rounded corners at 50% opacity
