The blue coloration on these tachi blades comes from a controlled thermal oxidation process applied during or after forging. When carbon steel is heated to specific temperature ranges and then allowed to cool under carefully managed conditions, the surface develops a thin oxide layer that refracts light at the blue end of the spectrum. This is distinct from chemical bluing used on firearms — on hand-forged blades, the tone tends to be more variable and organic, which is considered part of the aesthetic value. The finish also provides a mild degree of surface protection against humidity, though regular light oiling is still recommended for long-term display. The depth and evenness of the blue tone can vary slightly between individual pieces, making each one genuinely unique as a collectible.