Superwoman doesn't really do anythingfor me. She seems kinda bland.
Blackfire is interesting because on the surface she's starfire but more socialised and without the goofy semi-retarded alien-puppy aspect.
In her opening episode on OG-titans, she shows up as a friend and gets instantly accepted by the titans. It was actually an attempt to scapegoat her sister for her crimes.
Still, she passed herself off as friendly so well that if she'd have lied about why the bounty hunters were chasing her instead of just attacking her sister, she'd probably have gotten away with using Earth to run from the spacecops.