Carbon steel knives should be hand-washed with mild detergent and dried immediately after washing. They should never be washed in the dishwasher because dishwasher detergent is too harsh for the blades.
Carbon steel knives are prone to rust, so it is important to always thoroughly dry a carbon steel knife before putting it away to prevent rust from forming. Should rust form, use a kitchen scrubber and warm, soapy water to scrub it away.
Color changes in the blade of a carbon steel knife are common and can be expected. Despite proper care, a blade may turn gray or even black with repeated use.