The black oxide finish on Leatherman tools is a "conversion coating," which alters the surface of the metal rather than covering it like paint. During the process of oxidizing the outer layer of atoms on the surface, a black substance is formed that provides the black finish. This substance may wear away over time with regular use, but this is expected with this type of finish. To avoid staining, you can wipe down the tool surfaces with CRC 3-36 or a similar oil, which will help set the finish and make it last longer. This is similar to how your skin oils can help maintain the finish. Therefore, it is unlikely that you received a used tool by accident.