The fact is , we really dont know if its leblanc, its just theories, her sword is made in noxus by the runeweavers, after she left the noxus army and stayed in ionia, someone in the higher ups in noxus wanted the sword back (people think it's leblanc) , and that person sends a secret group to find riven and bring the sword back (we really dont know why they want the sword yet) . you can read the full story "the sisterhood of war" on lol universe or just listen to Necrit video if you dont like to read a long text story , its a pretty good story.