High carbon steel lacks the alloying elements that give “stainless” steel its name. This means that it is more susceptible to chemical interaction with its environment. Acidic foods like citrus, apples, onions, and tomatoes will cause the surface of the steel to oxidize and darken. This is completely normal, and in my opinion it is part of the beauty of high carbon steel. Over time, your blade will develop a patina of use that reflects the work it has done.
If you would like to refresh the surface of your steel, scrub it gently with a thick paste of baking soda and water. The paste will act as a light abrasive and will lift the oxides away. (Note: in the case of damascus steel, this process will also reduce the contrast of the pattern. This contrast is produced by essentially the same type of oxide layer, so abrasion will scrub some of that away too.)