If your knives are made from high carbon steel and you put the knife away with moisture still on the blade, you'll get dark stains on the wood from the steel reacting with the trapped moisture. This is especially true with white oak, which contains more tannins than our other species. Make sure you dry your knives completely before putting them back on your Mag-Blok. To remove the stains, block sand the face of the Mag-Blok with 150 grit sandpaper until the stains are gone. Further smooth the surface with 180 and 220 grit sandpaper, then apply a fresh coat of Blok-Butter.