Perhaps this goes without saying, but black ice is not actually black. It is clear. Maybe it is the dark-colored road surface visible through the clear ice that gives it its name. Maybe it is because it is so dangerous to people driving at night. Whatever the reason for the name, black ice is clear, not black. It is clear because it forms as a smooth layer on the road surface, making it as difficult to see as it is to drive on.
Black ice can form in different ways. A light freezing drizzle or mist can gradually coat a frozen road surface. Also, wind-blown snow can act as a polishing abrasive. In either case, vehicle tires can help to further polish the ice over time, which explains why black ice conditions can suddenly develop with little warning to the unsuspecting driver.