From what I can see, you have two things going on here, the typical dark patina that forms on carbon steel over time, and some pitting at the grain boundaries. The first is not a problem since this type of oxidation actually forms a protective layer on the steel surface. The pitting, however, is somewhat of a concern since over time and with exposure to moisture, these pits may continue to grow. Fortunately, they do not appear to be on or near the edge so you can probably leave them as they are, but I would definately coat the blade with a light film of mineral oil between uses.