Blackened copper with hydrochloric acid

Why does copper blacken when hydrochloric acid is rubbed onto it?

Joan,

My first question to you would be, What material are you using to rub the hydrochloric onto the copper? Sounds like you might be using a piece of material or fabric which may either contain cotton or some dye which is leaching onto the copper surface. When hydrochloric acid is dropped on a copper surface, the typical result is for the copper to lighten in appearance. If you are applying hydrochloric acid to copper with a cloth of some sort, I would imagine the source of the black exhibit would be coming from the material you're using. Could be dye, could be oxygen in the material which is raising atoms of copper on the surface to CuO, which is black.