My experience has been rendering your foreground over black then compositing the background in a photo-editing program is easier and more flexible. If you render the foreground and save it as a .psd or .tif file, the alpha channel will be included with the saved image. You can then use the alpha channel as a mask to clear the areas in the foreground where the background should show. With the background image on a separate layer you can then apply filters, blur or sharpen etc. without worrying about damaging the foreground image. Just my opinion ...