To make the navigation bar transparent, set the Translucent property of the UINavigationBar:

this.NavigationController.NavigationBar.Translucent = true;

To return it to the default, set the property to false:

this.NavigationController.NavigationBar.Translucent = false;

When the navigation bar is transparent, content in the underlying view can be seen beneath it. This means the visible content area expands to include the area ‘underneath’ the navigation bar.

The navigation bar does not allow you to interact with the controls underneath, so avoid placing buttons or other input controls completely beneath it.