导航条和view颜色如果设置一致，就会看到很明显的黑色横线，影响美观，所以需要隐藏掉，代码如下：

// 隐藏导航栏下方的黑线
    UIImageView *navBarHairlineImageView;
    navBarHairlineImageView = [self findHairlineImageViewUnder:self.navigationController.navigationBar];
    navBarHairlineImageView.hidden = YES;

- (UIImageView *)findHairlineImageViewUnder:(UIView *)view {
    if ([view isKindOfClass:UIImageView.class] && view.bounds.size.height <= 1.0) {
        return (UIImageView *)view;
    }
    for (UIView *subview in view.subviews) {
        UIImageView *imageView = [self findHairlineImageViewUnder:subview];
        if (imageView) {
            return imageView;            
        }
    }
    return nil;
}

参考：
iOS中自定义导航栏底部横线