The black thing rubbed out by stainless steel is a chemical polishing material, called Qingla, which is a kind of wax product. It is toxic, and long-term ingestion may cause cell abnormality, listlessness, memory loss, etc., and even cause cancer when severely beaten. Fortunately, this will gradually fade away with continuous use, and a small amount in the short term will not matter. Wipe it clean, you can't wash it off, so you must wipe it hard.