public double Transparency {get; set;}

//This example references a graphic element on a layout and sets its Transparency property (which is not available in the managed API)
//by accessing the element's CIMGraphic.

//Added references
using ArcGIS.Core.CIM;                             //CIM
using ArcGIS.Desktop.Core;                         //Project
using ArcGIS.Desktop.Layouts;                      //Layout class
using ArcGIS.Desktop.Framework.Threading.Tasks;    //QueuedTask

public class GraphicElementExample1
{
  public static Task<bool> UpdateElementTransparencyAsync(string LayoutName, string ElementName, int TransValue)
  {
    //Reference a layoutitem in a project by name
    LayoutProjectItem layoutItem = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(item => item.Name.Equals(LayoutName));
    if (layoutItem == null)
      return Task.FromResult(false);

    return QueuedTask.Run<bool>(() =>
    {
      //Reference and load the layout associated with the layout item
      Layout lyt = layoutItem.GetLayout();

      //Reference a element by name
      GraphicElement graElm = lyt.FindElement(ElementName) as GraphicElement;
      if (graElm == null)
        return false;

      //Modify the Transparency property that exists only in the CIMGraphic class.
      CIMGraphic CIMGra = graElm.GetGraphic() as CIMGraphic;
      CIMGra.Transparency = TransValue;             //e.g., TransValue = 50
      graElm.SetGraphic(CIMGra);

      return true;
    });
  }
}

//Update an element's transparency using the CIM.

//Perform on the worker thread
QueuedTask.Run(() =>
{
  // Reference and load the layout associated with the layout item
  Layout layout = layoutItem.GetLayout();
  if (layout != null)
  {
    // Reference a element by name
    GraphicElement graphicElement = layout.FindElement("MyElement") as GraphicElement;
    if (graphicElement != null)
    {
      // Modify the Transparency property that exists only in the CIMGraphic class.
      CIMGraphic CIMGraphic = graphicElement.GetGraphic() as CIMGraphic;
      CIMGraphic.Transparency = 50; // mark it 50% transparent
      graphicElement.SetGraphic(CIMGraphic);
    }
  }
});