I'd always assumed the moisture for black ice just came from the weather — it rains a little, then it freezes, and voila. But that's not the case. Black ice, in case this is a regional colloquialism that doesn't translate everywhere, is actually transparent ice. It's a thin layer of slippery stuff that forms on roads and is almost imperceptible to the eye. You look and see a normal road. You don't see the ice.