Nichirin Sword

Nichirin Blade is the name for the swords used by the Demon Killing Corps. Due to changing color depending on who their owner is, they are also known as The Blade of Color Change and in the anime The Color Changing Sword.

Description
While the blades look like normal swords, they have many special properties. The metal the blades are made of is the only mineral that is capable of cutting through a demon’s neck and decapitating them. This is due to the metal constantly absorbing sunlight. The swords take different colors depending on the user. Zenitsu has an interesting blade, since the yellow color of his blade isn't fully covering the sword. This may come from his cowardly personality, but this hasn’t been fully confirmed.

Forging Process
Nichirin Blades are forged with Scarlet Crimson Iron Sand and Scarlet Crimson Ore, both of which are found on the Sunlight Mountain, which is perpetually bathed in sunlight all throughout the year.

Blade Colours
Nichirin Blade change colours depending upon the user and that makes them unique and special to each user, as each has a unique properties. They can be of many colours.

Black Blades
This person with black blade symbolize that the person has deep and tragic life and has yet to find himself.

It also means that he's not really sure about the type of nature he is truly e.g. flames, water etc. Black

Blade users can have more than one type of nature of blade, such as Tanjiro Kamado can use water type technique as well as flame techniques.