Blacksteel

Blacksteel was a low-grade Rokugani steel, dusted with corrupted obsidian during the forging process which would enhance the damage dealed. The taint carried was minimal, and the weapons forged with this material din not radiated obvious taint. However, those who carried blacksteel items for long periods of time succumbed more easily to poison, sickness, and the Shadowlands Taint.

Illegal
The cheap, easy to produce, and powerful material had become popular with ronin and bandit gangs. The Kuni Witch Hunters destroyed any blacksteel weapon they discovered and their swordsmithers were arrested and executed.