It brings to mind the fly-by-night private services who can be defined as questionable, at best, when it comes to their patient care, billing practices, and every other aspect of their operation. I think the biggest difference, however, may be that it sounds like the black ambulances make absolutely no effort to appear legit, where the services I was referring to will jump through just enough hoops to skate by for a while without being detected by officials.
I spent a month working in the oil field in China last year, and I can tell you pretty much everything out there (especially in small towns) is scary. The oil field equipment was poorly put together exact rip offs of American and Canadian equipment with no safety equipment installed at all. We were trapped and held ransom by a local village for what turned into about 17 dollars, and there is 0 sanitation what so ever anywhere. The area which I was in the hepatitis rate among villagers was over 60% infected, and they used human waste on the fields, which increases the rate daily. I am really happy I don't work their full time in either the oilfield or EMS.