using System.Windows;
using System.Windows.Documents;
using System.Windows.Media;

namespace WindowManagement
{
    /// <summary>
    /// Adorner для контрла, с помощью данного контрала можно разместить поверх любого контрола любой другой.
    /// </summary>
    public class AdornerContainer : Adorner
    {
        private UIElement _child;

        protected override int VisualChildrenCount => _child != null ? 1 : 0;

        public UIElement Child
        {
            get
            {
                return _child;
            }
            set
            {
                AddVisualChild(value);
                _child = value;
            }
        }

        public AdornerContainer(UIElement adornedElement)
            : base(adornedElement)
        {
        }

        protected override Visual GetVisualChild(int index)
        {
            if (index == 0 && _child != null)
                return _child;
            return base.GetVisualChild(index);
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            _child?.Arrange(new Rect(finalSize));
            return finalSize;
        }
    }
}

using System.Windows;
using WindowManagement;

namespace WpfApp5
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            BlurWindowHelper.Blur(this);
        }
    }
}

using System.Windows;
using System.Windows.Documents;

namespace WindowManagement
{
    public static class AdornerHelper
    {

        public static void CreateElementAdornment(UIElement target, UIElement element)
        {
            //var contentPresenter = GetWindowContentPresenter(window);
            //if (contentPresenter == null) return null;
            var adornerLayer = AdornerLayer.GetAdornerLayer(target);
            var adornerContainer = new AdornerContainer(target) { Child = element };
            adornerLayer.Add(adornerContainer);
        }

        public static AdornerContainer GetElementAdornment(UIElement target)
        {
            //var contentPresenter = GetWindowContentPresenter(window);
            //if (contentPresenter == null) return null;
            var adornerLayer = AdornerLayer.GetAdornerLayer(target);
            var adorners = adornerLayer.GetAdorners(target);
            if (adorners == null || adorners.Length < 1) return null;
            return (AdornerContainer)adorners[0];
        }

        public static void RemoveAdorner(UIElement target)
        {
            var adornerLayer = AdornerLayer.GetAdornerLayer(target);
            var adornerContainer = GetElementAdornment(target);
            adornerLayer.Remove(adornerContainer);
        }
    }
}

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Media.Animation;

namespace WindowManagement
{
    public static class BlurWindowHelper
    {
        private static UIElement GetWindowContentPresenter(Window window)
        {
            //Находим корневой визуальный объект
            return VisualTree.FindChild<ContentPresenter>(window);
        }

        public static void Blur(Window window)
        {
            window.IsEnabled = false;
            var windowContentPresenter = GetWindowContentPresenter(window);
            //Viewport="0,0,4,4" ViewportUnits="Absolute"  TileMode="Tile" Stretch="Fill"
            var border = new Border
            {
                Background = new DrawingBrush
                {
                    Viewport = new Rect(0,0,4,4), ViewportUnits = BrushMappingMode.Absolute,  TileMode = TileMode.Tile, Stretch =  Stretch.Fill,
                    Drawing = new GeometryDrawing(Brushes.Transparent, new Pen(Brushes.Black, 1.0),  Geometry.Parse("M0,4 L4,0z"))
                },
                Opacity = 0.0
            };
            
            AdornerHelper.CreateElementAdornment(windowContentPresenter, border);

            var animation = new DoubleAnimation
            {
                To = 1.0,
                Duration = TimeSpan.FromSeconds(0.3)
            };
            border.BeginAnimation(UIElement.OpacityProperty, animation);
        }

        public static void Clear(Window window)
        {
            var windowContentPresenter = GetWindowContentPresenter(window);
            var container = AdornerHelper.GetElementAdornment(windowContentPresenter);
            var border = (Border)container.Child;
            var animation = new DoubleAnimation
            {
                To = 0.0,
                Duration = TimeSpan.FromSeconds(0.3)
            };
            //По завершению анимации удаляем Adorner и удаляем все 
            animation.Completed += (sender, e) =>
            {
                AdornerHelper.RemoveAdorner(windowContentPresenter);
            };
            border.BeginAnimation(UIElement.OpacityProperty, animation);
            window.IsEnabled = true;

        }
    }
}