Both Sia written songs. Although "Black and Blue" was never released it sounds to me like it could be one of the many songs that Rihanna rejected.

So which do you think was the better song for Rihanna: "Sledgehammer" or "Black and Blue"?
(Link to Black and Blue: http://youtu.be/rYV_JNzy6hQ )

Black and Blue is far better I'd say.

Both. Rihanna should go in this direction for the next album.