About the casting issues of swords and Japanese swords

The knowledge involved in this question is very complex, so I will briefly mention it here.

1. Knives or swords are not cast (cast steel, cast iron or other cast metals have extremely poor elasticity and toughness), they are made using multiple forging processes. After the sword is polished, the "moire" and "ground texture" on the surface are traces of multiple folding and forging, see the picture.

2. Knives or swords cannot directly cause black color, because no metal appears black when the surface is polished. Steel is called a "black metal" simply because it has a relatively low reflectivity, and it still looks like a mirror after polishing.

3. The black blade is the result of modern processes designed to improve corrosion resistance, such as carburizing, sandblasting oxidation, anodizing, etc. The best appearance is anodized black. However, in this process, the edged sword is put into the electrolytic tank as an anode, which will undoubtedly affect the sharpness of the blade. As for the character equipment in the anime, that's purely for visual effects.

4. Ancient weapons all relied on fine grinding (fine grinding) as the final process, so there could be no black-edged swords. To tell a joke, at that time, if a soldier's sword did not shine, it could only mean that he was lazy and the blade was rusty.