Black ice is just clear ice, with a somewhat shiny surface, over a black undersurface such as the road or other pavement. Doesn't really matter where it came from, a car tailpipe, refrozen runoff, freezing fog or drizzle. It can be hard to see, but it is not invisible. I encounter the stuff all the time, and have never fallen as a result, because I can see it. It's not actually invisible.