How would you go about retrieving this when using it with a Blake’s hitch and no eye at the end of your rope?

Depends on how you are using the uSAVER. If you are using the large ring and the pulley you would simply tie an overhand knot and that would pass through the large ring and catch on the pulley. 

If the pulley is through the large ring installed at the top of the tree you would girth hitch (with a couple of wraps turing it into a prusik) on the retrievel ball about 2-3ft from the end of the rope. This will slip through the pulley and get stuck on the small green ring at the end of the sling.