How to apply effect inside WPF UIElement OnRender method?

Question

I want to apply a blurr effect to a specific geometry, inside WPF UIElement OnRender method.

I have code like below.

        protected override void OnRender(DrawingContext drawingContext)
        {
            DrawingVisual drawingVisual = new DrawingVisual();

            using (var dc = drawingVisual.RenderOpen())
            {
                dc.DrawGeometry(null, _borderColorPen, _textGeometry);
            }

            drawingVisual.Effect = new BlurEffect { Radius = 5 };

            //  drawing geometry with blurr
            drawingContext.DrawDrawing(drawingVisual.Drawing);

            //  drawing geometry without blurr
            drawingContext.DrawGeometry(_foreColorBrush, null, _textGeometry);
        }

But blurr effect does'nt work.

Why is it? And what is the best way to do something like this?