Low-key lighting works well with a dark background. Direct the lights so that no light falls on the background. Position your main light, with either a small softbox or a wide bowl reflector, behind the model, lighting the back of the hair, the side of the neck and the outline of the shoulder, with a secondary light illuminating the side of the model's face. Half the model's face will be in shadow.
Low-key lighting produces a mysterious, intimate style that is well suited for romantic portraits of women. It can also be used for darker portraits of men, but use it without a softbox to produce a harder look.