This mornings first attempt with black muslin background..
and the end result which requires a lot more photoshop than the white background, possibly because I did not use the backlight to get her hair outline lit up in the shot.
Also, any dirt or blemish on the black just picks up light like that... so the trick seems to be to place the model farther away from the background when it is black.
Have to stop the experimenting for now and get back to work, errands etc.