i'm probably telling you something you already know, but just put your subject as far from the BG as possible. even though it (the BG) will be black, it can sometimes pick up some stray light if it is too close to the subject. then, put your lights anywhere you want them for the desired sculpting of light on your subject.
And you can add a flag to let the light hit your talent but to keep it from hitting the background. So if your light is to the side of the person, block off the light from hitting the background with an additional card such as foam core, or matte board placed between the fixture and the black background but out of the shot.