My AP textbook says that the US had segregated ambulances for decades. If this is true, how would it even work? If there were no black ambulances in range, would they just leave you to bleed out?

Basically yes. Funeral homes provided the only transports in many areas to go to the hospital. Many people died waiting on family or a black funeral home or ambulance to transport them. There was also no medical care on most trips to the hospital until the mid 1970s when ambulances in some areas started carrying nurses and the idea of emts were introduced. Also segregated hospitals were notoriously inferior. My father remembers being a mechanic and working on the funeral directors wagon to make it faster. He's told me everyone was afraid to be on the road with that maniac but he sure applied what we now call "diesel therapy" quite proficiently.