By using this function you can flash your selected feature.

private void HighlightIFeature(IActiveView activView, IFeature pfeature)
{
 IFeatureIdentifyObj featIdentify = new FeatureIdentifyObject();
 featIdentify.Feature = pfeature;
 IIdentifyObj identify = featIdentify as IIdentifyObj;
 identify.Flash(activeView.ScreenDisplay);
}