Black Blade

The Black Blade is a katana like weapon wielded by Black★Rock Shooter.

Black ★ Rock Shooter OVA
Black Blade is one of the weapons of choice used by Black★Rock Shooter. However, the only time we see it is in the fight between Black★Rock Shooter and Black★Gold Saw. Black Blade was probably lost in the fight, but whatever the reason, we don't see it again.

Black Rock Shooter (2012)
In the anime, Black Blade was seen again while Black★Rock Shooter was fighting Chariot. Like the characters returning from the OVA, it seems to have been redesigned as well. It was used to behead both Chariot and the hooded figure that seems to resemble Arata Kohata. It was also used to kill Dead Master later on.

Abillities
It is used like a regular katana. Its attacks aren't much, but it still deals an amount of damage. In the anime, it was used for beheading and killing certain characters.