ALL cars should probably have a blackbox now, the technology is there and it wouldn't be too expensive. Human drivers are very unreliable witnesses when trying to establish the reasons for an accident. They will under estimate speed (or lie about it) and their perception of their movement and where other vehicles were is often just plain wrong.
In the case of autonomous cars, the occupant will be even more unaware of what is happening around them. So a recording system of the data available will be helpful in establishing the reasons for a crash.
This is not an admission that the driverless technology is dodgy. If we follow your aircraft anology, the blackbox there sometimes finds pilot error. But it can also find other reasons for a crash - foul play, adverse weather conditions, a parts malfunction. We accept black boxes on aeroplanes, why would we object to them in cars?
Often, if you are involved in a crash, it is not your fault, it is the other driver. The black box on an autonomous car will be able to relay information for use by the investigating authorities, which will more likely established that the computer driven vehicle will be superior to the human drivers around it.