The blue finish on these ninjato blades comes from controlled oxidation or chemical bluing processes applied after the blade is ground and heat-treated. In controlled bluing, the steel surface is exposed to specific chemical solutions or heat cycles that produce a stable iron oxide layer - magnetite (Fe₃O₄) - which registers as a deep blue or blue-black depending on thickness and steel composition. This layer is not paint or coating in the conventional sense; it bonds at the surface level and offers a degree of corrosion resistance while preserving the underlying steel's structural properties. On high-carbon steel blanks like 1095, the finish tends to show more depth and contrast, especially near the hamon line. On manganese steel, the finish appears more uniform and slightly darker. Regular light oiling helps maintain the finish quality over time, particularly in humid storage environments.