Xaml doesn't have bitmap effects. You'll need to interop with DirectX for sharers.
I haven't tried it, but for static elements you may be able create an element which renders the elements underneath it, passes that to DX to blur, and then displays that on top.
For a MediaElement you would need to write an MFT to apply your filter. See
http://code.msdn.microsoft.com/windowsapps/Media-extensions-sample-7b466096
--Rob
