No, Blend only works one way.

What IB allows you to do is to subclass UIView, design your own controls in code and preview the results of changing instance variables directly in IB.

You can do that by implementing designer aware controls since .NET 1.0, so I don't see the difference, other than having a bit extra code maybe.

I suppose you are referring to this WPF functionality https://docs.microsoft.com/en-us/dotnet/framework/wpf/contro..., and similarly in UWP: https://docs.microsoft.com/en-us/windows/uwp/design/controls...? While it looks powerful, I have to say that the Xcode interface builder and code annotation approach has a lot less friction.

Yes, exactly that, but you don't need to go the full way.

It depends on what kind of rendering one wants to present to the user/developer, even with DesignerAttribute one can provide a custom rendering for the control.

Here with a forms example, but similar way can be done for the XAML variants.

https://msdn.microsoft.com/en-us/library/ms171824.aspx