There is more than one way to do it:
1- Add your UIView
on UIWindow
instead of adding it on UIViewController
. This way it will be on top of everything.
[[(AppDelegate *)[UIApplication sharedApplication].delegate window] addSubview:view];
2- Use custom transition that will keep your UIViewController showing in the back with a 0.5 alpha
For that I recommend you look at this: https://github.com/Citrrus/BlurryModalSegue