Adding glass effect to listbox item

I have a stackpanel with a background image. Inside the stackpanel there is a ListBox and its background is set to transparent, what I need is to blur (glass effect) the background image only the part that comes under the listbox item. I tried to add a rectangle to transparent color in the listbox item template and then added a blurred effect but it doesn't work. This is what I am trying to achieve.

I came across this some time ago, maybe will help with what you're after.