they are all over china , basicaLLY THEY are unregistered taxis. usually unmetered

They are the flock of Chinese that nearly attack you to ride with them whenever you leave an airport, bus station, etc.

If it's your first time to an area and don't no the way or at least know a reasonable price to get to your desired location, I'd recommend not using the black taxis. Just walk past them while you look like you know where you're going, even if you just walk around the block to catch your breath and orientate yourself.