From http://stackoverflow.com/questions/19236410/back-button-strangely-disappearing-in-uinavigationcontroller-but-keeps-working

backIndicatorImage

[[UINavigationBar appearance] setTintColor:[UIColor grayColor]];
[[UIBarButtonItem appearance] setBackButtonTitlePositionAdjustment:UIOffsetMake(0, -60) forBarMetrics:UIBarMetricsDefault]; // Takes out title

UIImage *backButtonImage = [UIImage imageNamed:@"BackArrowDark.png"];

if ([UINavigationBar instancesRespondToSelector:@selector(setBackIndicatorImage:)]) {
  [[UINavigationBar appearance] setBackIndicatorImage:backButtonImage];
  [[UINavigationBar appearance] setBackIndicatorTransitionMaskImage:backButtonImage];
} else {
  int imageSize = 21; // REPLACE WITH YOUR IMAGE WIDTH

  [[UIBarButtonItem appearance] setBackButtonBackgroundImage:[backButtonImage resizableImageWithCapInsets:UIEdgeInsetsMake(0, imageSize, 0, 0)] forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
}