It forms when dew on the road turns to a thin layer of ice. It sux hardcore and as said, once you are on it you have no chance of controlling the car, it just slides wherever it wants until it hits something and comes to a stop, or you come off the ice. Its also next to impossible to see as its a very thin layer, often following the contours of the rocks in the road, which makes it hard to spot because its not flat and slick/reflective.
I was driving to Jindabyne a few years back and a birgde past Cooma airport was completely covered in black ice. We were doing about 50km/h (in a rated 100km/h zone) and as soon as you got on the bridge it was game over, no control what so ever, you jusrt hoped you could slide to the other side without hitting anything. Luckily I didnt as I was in the GTR, still a shit scary experience though.
Sux to hear about the car man