Now that we have the blocks created, we'll add a simple outline around them.
Duplicate the top layer (Layer> Duplicate Layer or Ctrl+J) and change the blending mode of the new layer to Screen.

Apply a Find Edges filter (Filter> Stylize> Find Edges).

Invert the current layer (Image> Adjustments> Invert or Ctrl+I) and the outline should blend into the image. One last thing to do is to reduce the opacity to 25%.