how to make custom popup (Overlay) with semi-transparent background?

I’ve got basic storyboard scene changes working fine.  Now I’m trying to implement custom popups on top of scenes, for things like tutorial info.  I learned about the storyboard overlay, and got a basic one of those working.  Although I don’t see how an overlay scene is any different than a normal scene.  I want the overlay to be smaller than the screen, so that the current scene is still somewhat visible under the overlay, but blurred or ‘glassy’.  I’m not talking about modal here either; I’ve got that working fine as well.

I thought it would be simple, by just setting the alpha of the overlay scene’s display group, like so (this is from the overlay scene):

-- Called when the scene's view does not exist: function scene:createScene( event )     local group = self.view     group.alpha = 1     local wrappedText = display.newText( "Explain stuff here", 5, 5, 150, 100, native.systemFont, 18 )     group:insert( wrappedText )     end  

but this isn’t working.  Clearly I’m missing something basic here.  Do I need to add some kind of transparent background image to the overlay that is full screen sized, and change the alpha of that?