Wow, great tutorial, thanks for sharing. And I understood it, here you go:
Just a small test scenario:
1) Select what you want the boarder from
2) Create a new layer, switch to it and fill the mask with black.
3) Deselect the mask and apply a blur on the newly created layer.
4) Now choose the magic wand with 0% tolerance. Select the (transparent) background and the black interiour (holding CTRL on the second click).
5) Now invert the selection. Here you go and just have the boarder selected.
6) Delete the created temp layer. You can now apply your blur (or whatever you want) just on the boarder.