public class BlurAroundAdorner2 : Adorner
  {
    private const double radius = 10;
    private Thickness thickness;
    private Brush brush;

    /// <param name="adornerElement"> элемент, вокруг которого создается рамка</param>
    /// <param name="thickness"> толщина рамки </param>
    /// <param name="effectSourceVisual"> источник картинки для размытого фона рамки </param>
    public BlurAroundAdorner2(UIElement adornerElement, Thickness thickness, Visual effectSourceVisual)
      : base(adornerElement)
    {
      if (adornerElement == null || effectSourceVisual == null)
        throw new ArgumentNullException();

      this.thickness = thickness;
      brush = new VisualBrush(effectSourceVisual);
      Effect = new BlurEffect() { Radius = radius };
    }

    // рендер
    protected override void OnRender(DrawingContext drawingContext)
    {
      var outsideOuter = new RectangleGeometry(
        new Rect(-thickness.Left - radius,
                 -thickness.Top - radius,
                 ActualWidth + thickness.Left + thickness.Right + 2 * radius,
                 ActualHeight + thickness.Top + thickness.Bottom + 2 * radius));
      var insideOuter = new RectangleGeometry(
        new Rect(-thickness.Left,
                 -thickness.Top,
                 ActualWidth + thickness.Left + thickness.Right,
                 ActualHeight + thickness.Top + thickness.Bottom));
      var insideInner = new RectangleGeometry(new Rect(0, 0, ActualWidth, ActualHeight));
      var inside = new GeometryGroup() { Children = new GeometryCollection() { insideInner, insideOuter } };
      var outside = new GeometryGroup() { Children = new GeometryCollection() { outsideOuter, insideInner } };

      Clip = inside;
      drawingContext.DrawGeometry(new SolidColorBrush(Colors.White), null, outside);
      drawingContext.DrawGeometry(brush, null, insideOuter);
      
    }
  }