Anyone who has used a carbon knife can testify that they are awesome. They can get razor sharp, they cut beautifully and are easy to sharpen.
One characteristic of carbon steel knives is that they can rust if not cared for properly. To avoid rusting, please wipe the blade clean immediately after use and dry thoroughly. Do not store a knife that isn't dry.
With time and proper care, your knife will develop what is called patina. The patina on your blade is a beneficial layer that actually protects against rusting.
Cool colors like blue and grey are good, hot colors like red and orange are bad. If you do happen to notice a little rust forming, not to worry. Simply use a soft sponge or gentle scrubby (not steel wool) with warm soapy water to remove.