Knives with a high iron content that have not developed a patina will produce a black stain on wood, even if thoroughly dried before placing them on the holder. The most likely cause is that knives with small amounts of free iron on the surface can be affected by anaerobic bacteria that are able to create magnetite in oxygen-free environments. They do this by reducing ferric oxide (rust) to magnetite, which is deposited on the surface of the wood and is attracted to magnetic force. This action is accelerated when the knife holder is in a humid environment, such as a steamy area of the kitchen.
Also, tannic acid in wood is corrosive to ferrous metals (irons) - knives of carbon steel that are not stainless. This results in a blue-black stain on the wood. The knife in this image produced all visible stains. Black smudges will not necessarily take the shape of the knife in question, and don't just appear over the magnetized area. While such black smudges are rare, be sure to use your knife holder only for cutlery that has a stainless finish.
Fortunately, these types of stains are relatively shallow. To correct the problem, your knife bar will need to be sanded with 320 grit sandpaper. This is best done with a drum sander or orbital sander on the entire surface. This can be done at wooDsom's factory if you choose. Your knife holder will be returned in its best possible condition with a new mounting kit and wooden plugs. If you experience this problem, we recommend applying a thin urethane finish to the knife holder after it has been reconditioned.