Ancient Japanese legends tell of a strange blade, a relative of the katana, that has its razor sharp edge on top of the curve instead of below, a reverse blade which is meant to confound the enemy and make it harder for them to defend against in combat.
This blade is called a sakabato, and although it has been depicted in classic samurai fiction, anime series like Rurouni Kenshin, and recreated by geeky replica weapon designers it was thought to be nothing but a legend until a sakabato from the Edo period was discovered inside a storage locker. It appears not all legends are based on pure imagination, or else there's an incredibly talented, and nerdy, blacksmith out there making anime dreams come true!
Via Nerd Approved
There had been double edged swords too...
Also, at only around 20 cm length it would be more appropriately called a Tanto (15-30 cm), or short-blade/dagger/knife. The two typical sword lengths being around 60-73 cm (katana) or 30-60 cm (wakizashi).