Over time, carbon steel blades gradually discolour from use and a dark patina forms. It is completely harmless and has no effect on how the knife functions.
However if you wish, the blade can be brightened with “Vienna chalk” or a scouring agent and a coarse sponge. This returns the blade nearly to its first shine.
The same effect is also very successfully achieved with our “rust eraser” which removes rust or discolouration from the blade. The eraser can be used dry or even more effectively with a little water.