You picked a really challenging outfit and background to work with here, and you did a nice job. Particularly with only one light. An easy way to avoid losing her against the background is to have it (or her dress) be a different color. The one with the pink shirt is a good example.
You already have the idea down of controlling your spill light, so think about getting a lighter background and controlling its color by how much light exposure it gets (start with something at a middle grey range and brighten or darken it with exposure control).
For this particular setup, with the black on black, I would try a rimlight not from above, but directly behind the model. It would give you a well lit halo of edge light that would give separation from the background and still keep the black on black look. I notice you have a speedlight listed in your gear on flickr -- can you trigger it off camera? If so that would be an easy thing to hide behind her.
Use of fill is good here, it looks to me like you got the V-card nice and close to catch plenty of light and get a nice ratio of main to fill.
Your girlfriend is great model, too! Hope you'll share more of these with us soon.
