Ice or freezing rain becomes black ice when raindrops that are not freezing, in other words rain that has a temperature above 0° C, hits cold ground and freezes. This occurs in temperate zones and sub polar areas in winter.
Black ice has two tricky features that make it so treacherous. One minute the street is dry and in a good condition to drive on. Drivers do not expect that when it begins to rain, ice can instantly accumulate on the road. However, when the temperature drops to around freezing, black ice can be expected on bridges and in other cold places.

The ice warning system in cars which has become standard equipment and which switches on at 3° C warns drivers of impending ice and should therefore be taken seriously when rainfall starts. When conditions have deteriorated completely, the vehicle loses its grip, making it impossible to steer. Adjusting one's speed is therefore an absolute must so that potential damage can be contained.
The second unpleasant trait of black ice is extreme slipperiness. Whereas  parts of the precipitation during ice rain or hail comes down as ice or hail and therefore have a rougher surface, water during black ice can spread before it freezes, which is why the surface it hits becomes as smooth as glass and winter tires and even winter shoes lose their grip. Only spikes can improve the situation, but in Germany spikes are against the law except for a very small area near the border to Austria. The damage spikes do to dry streets and also the fact that they become dangerous on dry streets do not justify using them, due to the fact that black ice thankfully does not occur all that often.

Black ice does not only handicap traffic on roads, it also handicaps railway  and airway traffic. Ice can form on the overhead wire of the railway systems. When it does, there are short periods of interruptions between the collector and the overhead wire which can lead to electric arc effects. While the collector removes the ice (supported by the vibrations in the overhead wire associated with the movement) so that operation itself must not be halted, black ice decreases the speed and increases the abrasion of the wear strip.
Black ice can form on the wings of planes, making the plane heavier and also changing the profile of the wing, which is important for the ascending force of the plane. That is why a special spray is used to clear airplanes of ice before take off. The danger is thus diminished for take-off but there have been reports of accidents, some of which seem to be attributable to black ice during the flight.