A number of states recognize the "sudden medical emergency defense," which can relieve a driver from liability when they suffer an unforeseen medical emergency that causes a car accident. In these states, if you black out and cause a car accident, you may be able to avoid financial responsibility, but you'll need to prove a few things first.
A driver who claims to have blacked out has the burden of proof to show that they lost consciousness before the accident occurred, that the loss of consciousness caused the loss of control of the vehicle, and that it was an unforeseeable medical emergency.
The issue of "foreseeability" usually raises the most questions when determining whether the sudden medical emergency defense applies to a black out.
When a driver has no history of black outs, but then suffers one while driving, they may be able to successfully assert the defense, since there was no reason to anticipate the incident. On the other hand, if a driver has a history of blacking out -- or has an underlying condition that is known to cause these episodes from time to time -- they will have a much tougher time successfully arguing that the sudden medical emergency defense applies, since the black out could certainly be deemed foreseeable in that situation.
It's a practical fact that the "sudden medical emergency" defense can leave a car accident victim with no recourse against the person who technically caused the accident. But that's not to say that the injured person will be left with no recourse at all. If you live in a "no-fault" car insurance system, your own insurance will pay for your injuries and other losses (up to a certain financial threshold) after a car accident, regardless of who or what caused the crash. And in other states, as long as you have your own car insurance policy in place, you can almost always turn to that coverage to pay your medical and car repair bills.