What weather reports call black ice is not black at all; it just looks that way because it is transparent and whatever color the road’s surface is shows through.
Whether ice on the road is visible depends on how it is formed. Rime ice, which is visible, forms when droplets of freezing drizzle adhere to the roadway with air trapped inside because the drops do not splatter when they hit.
Black ice, which is not easily seen, is a coating of thin ice that is often the result of thawing and refreezing roadside snowbanks. It may also come from freezing drizzle or rain that splatters when it hits the road, so that no air is trapped inside.
In daylight, black ice may look like a dull patch of road surface that is darker than the rest of the highway. It is most treacherous at night, when it is difficult or impossible to spot. Unfortunately, it is also most common at night, because that is when temperatures drop, traffic flow eases, and melted snow refreezes.
Black ice should be suspected, and driving speeds should be reduced, when temperatures are near or below freezing, especially in fall and early winter, when there is no obvious precipitation on the road.