更改UIAlertcontroller背景颜色

好的，这样我就收到了我正在使用的警报，我希望它的背景是黑色而不是灰色。我设法更改了标题和消息的文本颜色，但没有更改背景颜色。很好达到我想要的颜色。我将其更改为绿色，蓝色和白色，但未更改为黑色。当我尝试将其更改为黑色时，它会变成灰色。任何建议都会有所帮助，我们将不胜感激。我在这里尝试过如何更改UIAlertController的背景颜色？这就是我到达现在的位置的方式。

这是我现在要去的事情：

func showAlert(title:String, message:String) {

    //Set up for the title color
    let attributedString = NSAttributedString(string: title, attributes: [
        NSFontAttributeName : UIFont.systemFontOfSize(15), //your font here,
        NSForegroundColorAttributeName : UIColor.whiteColor()
        ])
    //Set up for the Message Color
    let attributedString2 = NSAttributedString(string: message, attributes: [
        NSFontAttributeName : UIFont.systemFontOfSize(15), //your font here,
        NSForegroundColorAttributeName : UIColor.whiteColor()
        ])

    let alert = UIAlertController(title: title,message: message, preferredStyle: .Alert)

    alert.setValue(attributedString, forKey: "attributedTitle")
    alert.setValue(attributedString2, forKey: "attributedMessage")
    //alert.view.tintColor = UIColor.whiteColor()
    let dismissAction = UIAlertAction(title: "Dismiss", style: .Destructive, handler: nil)
        alert.addAction(dismissAction)
    self.presentViewController(alert, animated: true, completion: nil)
    //set the color of the Alert
    let subview = alert.view.subviews.first! as UIView
    let alertContentView = subview.subviews.first! as UIView
    alertContentView.backgroundColor = UIColor.blackColor()
    //alertContentView.backgroundColor = UIColor.greenColor()
    //Changes is to a grey color :( 
    /*
    alertContentView.backgroundColor = UIColor(
        red: 0,
        green: 0,
        blue: 0,
        alpha: 1.0)
    //Also another Grey Color Not batman black
    */

    //alertContentView.backgroundColor = UIColor.blueColor()
    //turns into a purple



}