第一種方法：

UIBlurEffect *effect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleLight];
UIVisualEffectView *effectView = [[UIVisualEffectView alloc] initWithEffect:effect]; effectView.alpha = 0.9;
effectView.frame = CGRectMake(0, 0, bgImageView.frame.size.width, bgImageView.frame.size.height);
[bgImageView addSubview:effectView];

第二種方法：

UIToolbar *toolbar = [[UIToolbaralloc] initWithFrame:CGRectMake(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT)];
toolbar.barStyle = UIBarStyleDefault;
[_backImg addSubview:toolbar];