So obviously, these are fitness photos, she will want to show off the definition of her body. I got a brand new black sheet I have been dieing to use, and this was the perfect time.
Human body = relatively bright compared to a black background, so she would stand out really well against this dark color. Also using two rim lights will further help punch her shape right into that black background.
So, three lights total. One hard strobe on each side and behind the model (10o'clock and 2o'clock - the model is in the center of the clock and I am shooting from 6o'clock) and then a final light high and to camera right (or the 5o'clock position) with a small softbox on it to soften the light.
It always helps to keep as much distance as you can between the subject and the background. Also, try to flag your rim lights in particular because they have a tendency to spill onto the background, and occasionally into the frame along the edges - and it doesn't look nice.
What else should I have done with my black background on this shoot? Probably used a vibrantly-gelled flash pointed at the background only, to get a colored spot. Its a quick way to add a little variety to your shoot. Unfortunately when I am shooting I have a tendency to think just from shot to shot. If I had this idea organized and on paper beforehand I am sure I would have shot a few frames like this.
What else did I learn? Always have your ideas written down to keep you thinking about the whole shoot when your in the middle of it.