You select the black parts, hit Ctrl+Shift+I to select everything else, make a new layer out of it, go to Filter>Blur>Gausian Blur (however it's spelled), set it to a 5 pixel radius. Next, set the layer-blending mode to "screen" and duplicate it 2-5 more times (depending on how bright the backlight is). I used a gradient tool to add some color (i.e. the purple around the edges). This can be applied when making other sillhouetted things...