您可以根据需要设置alpha，这将在任何情况下导致一些透明度。

UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"Alert title" message:@"Alert message" preferredStyle:UIAlertControllerStyleAlert];

UIAlertAction* ok = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:nil];
[alertController addAction:ok];

alertController.view.backgroundColor = [UIColor colorWithWhite:1 alpha:0.6];

[self presentViewController:alertController animated:YES completion:nil];

我是这样做的：

class alertController : UIAlertController{
       override func viewWillDisappear(_ animated: Bool) {
           super.viewWillDisappear(animated)
           self.presentingViewController?.view.alpha = 1
               
       }
       override func viewWillAppear(_ animated: Bool) {
           super.viewWillAppear(animated)
           self.presentingViewController?.view.alpha = 0.5

}
}