Beijing Black Cars
Black cars are not necessarily black. You can see them almost anywhere, near subway stations or main bus stations.

The owners of these cars are not licensed as cab drivers and by offering you their service as a cab they are doing something illegal.

Remember, there is no insurance and no receipt when you take a black car and if you leave something in the car you will not get it back. Most of the time, the price they are offering is not cheaper than what a real cab is offering and when it is cheaper it's still not worth the risk.