public bool IsLabelVisible {get;}

var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  // toggle the label visibility
  featureLayer.SetLabelVisibility(!featureLayer.IsLabelVisible);
});

//Note: call within QueuedTask.Run()

if (!featureLayer.IsLabelVisible)
  //set the label's visiblity
  featureLayer.SetLabelVisibility(true);