My knives are made out of high carbon steel, which means the blade is susceptible to rust without the proper care. It is natural that your blade will develop a darker patina with use.
To take care of your blade always keep it dry. Never leave soaking in the sink or put in the dishwasher.
Coat your blade every so often with oil. I use and recommend coconut oil because it is food safe and will protect against any rust.
If you feel like your blade is getting dull, sharpen it! I like to use a series of fine grit wet stones and finishing it off with a leather strop.
If you follow these simple instructions you will have a knife that will serve you for years to come.