typedef NS_ENUM(NSInteger, UIBlurEffectStyle) {
    UIBlurEffectStyleExtraLight,
    UIBlurEffectStyleLight,
    UIBlurEffectStyleDark
} NS_ENUM_AVAILABLE_IOS(8_0);


    UIBlurEffect *blurEffect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleDark];
    UIVisualEffectView *blurEffectView = [[UIVisualEffectView alloc] initWithEffect:blurEffect];
    blurEffectView.frame = self.imageView.bounds;
    [self.imageView insertSubview:blurEffectView atIndex:0];
 

未处理效果

UIBlurEffectStyleLight

UIBlurEffectStyleExtraLight

UIBlurEffectStyleDark

iOS8自带模糊效果