Kurosawa (黒澤, Literally meaning: Black Swamp) is the name of Mifune's sword. The sword is fairly long and thin, and has a rectangular hand-guard and a long hilt wrapped in bandages. Mifune has shown great skill and dexterity in wielding the sword, performing Iaidō with the blade. He also used it to slash open Ibuse's head when the salamander swallowed him.
Mifune refers to his sword as a Meitō (名刀, Literally meaning: Famous/Excellent Sword), which means that Kurosawa has a history and a legendary status, having become part of the identity of its owner.
The name of the sword is probably derived from the name of the famed samurai film director, Akira Kurosawa, who also directed the film Yōjinbō, where Mifune's name also comes from.
When Mifune clashed swords with Sasuke, the sword he wielded was not Kurosawa.