What is black powder, that collects only on plastic, when wiped off with it leaves a stain.?

It could be one of a number of things; but if you live in an urban area, it is most likely soot from car exhaust; which may contain small amounts of organic materials or sulfur compounds that bind to the surface of plastic. If you are in a cool area, where people burn heating oil, or even coal for heating purposes; the soot is even more noticeable. Because of static electricity, the soot is attracted to plastic surfaces.