Hi!
You can't set the properties of the callout shown using GeoView.ShowCalloutAt. You can, however, create a Callout object, set its display properties, and add it to the map view. Here's an example:
Callout customCallout = new Callout
{
Background = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Colors.CornflowerBlue),
BorderBrush = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Colors.PaleVioletRed),
BorderThickness = new System.Windows.Thickness(5,5,5,3),
Content = "Custom content goes here"
};
GeoView.SetViewOverlayAnchor(customCallout, mapLocation);
MyMapView.Overlays.Items.Add(customCallout);
I hope that helps! -Thad