Black oxide is a "conversion coating", which means it changes the surface of the product, rather than covering the product, as paint would. Black oxide is formed by oxidising the outer layer of atoms on the surface. In effect, it's controlled rust. When you first use the product, the black substance is this outer layer. It will wear away to some extent, but that's the nature of this finish. To avoid the stains, simply take a rag with CRC 3-36 (or similar oil) and rub the tool surfaces thoroughly. The oil will set the finish and allow it to last longer. This is what your skin oils are actually doing when you handle the tool.