Who is responsible when backing out of your own private driveway and hit a car parked in front of and obstructing the driveway?

It is the person who is blocking the private driveway's fault because they should have known not to park in front of a private driveway in the first place. However, if the private driveway is not marked with a sign of some sort to let people know that it is private, it may be the driveway owner's fault.