It was a legendary katana, forged at the same time as the Masamune and tested in the same stream as the Masamune. The Murasame was so sharp, that it cut leaves as they floated past on the stream. The Masamune was so sharp that it cut the water, and the leaves floated around past it.

thus, the Masamune was seen to be a more peace loving sword, while the Murasame was seen to be a more aggressive and bloodthirsty sword.