Exemplo de como destacar uma região específica no mapa com Xamarin.Forms http://julianocustodio.com/regioes-maps-xamarin-forms

CustomMapRendereriOS.cs

using UIKit;
using Xamarin.Forms;
using App.iOS;
using Xamarin.Forms.Maps.iOS;
using MapKit;
using Xamarin.Forms.Platform.iOS;
using CoreLocation;
using ObjCRuntime;

[assembly: ExportRenderer(typeof(global::App.CustomMap), typeof(CustomMapRenderer))]
namespace App.iOS
{
    public class CustomMapRenderer : MapRenderer
    {
        MKPolygonRenderer polygonRenderer;

        protected override void OnElementChanged(ElementChangedEventArgs<View> e)
        {
            base.OnElementChanged(e);

            if (e.OldElement != null)
            {
                var nativeMap = Control as MKMapView;
                if (nativeMap != null)
                {
                    nativeMap.RemoveOverlays(nativeMap.Overlays);
                    nativeMap.OverlayRenderer = null;
                    polygonRenderer = null;
                }
            }

            if (e.NewElement != null)
            {
                var formsMap = (global::App.CustomMap)e.NewElement;
                var nativeMap = Control as MKMapView;

                nativeMap.OverlayRenderer = GetOverlayRenderer;

                CLLocationCoordinate2D[] coords = new CLLocationCoordinate2D[formsMap.ShapeCoordinates.Count];

                int index = 0;
                foreach (var position in formsMap.ShapeCoordinates)
                {
                    coords[index] = new CLLocationCoordinate2D(position.Latitude, position.Longitude);
                    index++;
                }

                var blockOverlay = MKPolygon.FromCoordinates(coords);
                nativeMap.AddOverlay(blockOverlay);
            }
        }

        MKOverlayRenderer GetOverlayRenderer(MKMapView mapView, IMKOverlay overlayWrapper)
        {
            if (polygonRenderer == null && !Equals(overlayWrapper, null))
            {
                var overlay = Runtime.GetNSObject(overlayWrapper.Handle) as IMKOverlay;
                polygonRenderer = new MKPolygonRenderer(overlay as MKPolygon)
                {
                    FillColor = UIColor.Red,
                    StrokeColor = UIColor.Blue,
                    Alpha = 0.4f,
                    LineWidth = 9
                };
            }
            return polygonRenderer;
        }
    }
}