I am not really sure what do you mean by blurry, but if you want to set some kind of opacity you can set ti with setting ModalBackground like this:
window.ModalBackground = new SolidColorBrush(Color.FromArgb( 66, 255, 0, 0));
Where the first value of the FromArgb
method is the opacity. Note that you can't set background in WPF , but just in XBAP or Silverlight.
the Telerik team