方法一

在当前页面加入下面方法

- (void)viewWillAppear:(BOOL)animated{
    [super viewWillAppear:animated];
    [self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
    //去掉导航栏底部的黑线
    self.navigationController.navigationBar.shadowImage = [UIImage new];
}

如果仅设置当前页导航透明，需加入下面方法

- (void)viewWillDisappear:(BOOL)animated{
    [super viewWillDisappear:animated];
    [self.navigationController.navigationBar setBackgroundImage:nil forBarMetrics:UIBarMetricsDefault];
    [self.navigationController.navigationBar setShadowImage:nil];
}

需要注意的是：如果返回上一页的操作有动画，则在上一页出现的时候，导航栏的颜色会逐渐变化

方法二

- (void)viewWillAppear:(BOOL)animated{
    [super viewWillAppear:animated];
    [[self.navigationController.navigationBar subviews] objectAtIndex:0].alpha = 0;
}

仅设置当前页面

- (void)viewWillDisappear:(BOOL)animated{
    [super viewWillDisappear:animated];
    [[self.navigationController.navigationBar subviews] objectAtIndex:0].alpha = 1;
}

个人感觉：方法二比方法一更为简单，而且效果也好