Hmm, when do you move the black sprite to the front? I assume you’re doing it using go.set_position()? Is the black sprite in the loaded collection? You can probably wait for proxy_loaded, then do init and at the same time move the black sprite to front and then do enable?
By the way, have you considered using a full screen gui node instead to cover both gui and game objects (or maybe you have modified the default render script to take care of this)? And have you seen this post about transitions using stencil buffers?