SkiaSharp Modify and save existing bitmap

I am trying to take an existing fingerpaint bitmap which is loaded onto the canvas, modify it by adding fingerpainting to it, and then save the modified bitmap. When I reload the newly saved bitmap it comes up as all black. Can anyone help?

    void SaveSketchToFile_EventHandler(object sender, EventArgs e)
    {

        // find and delete any existing file from local database
        IEnumerable<DocumentFile> binarySketches = App.database.GetSketches(sampleStationPassed.SampleStationLocalGuid);

        foreach(DocumentFile sketch in binarySketches)
        {
            App.Database.DeleteImageFromDatabase(sketch, sampleStationPassed.SampleStationLocalGuid);
        }


        // and now save what is on the canvas....


        // create non-visible surface in memory
        var info = new SKImageInfo((int)canvasView.CanvasSize.Width, (int)canvasView.CanvasSize.Height, SKImageInfo.PlatformColorType, SKAlphaType.Premul);
        var surface = SKSurface.Create(info);


        if(bitmapSketch != null)
        {
            using (var existingCanvas = new SKCanvas(bitmapSketch))
            {

                existingCanvas.Clear(SKColors.White);

                // draw original bitmap
                // Draw original in full size
                float x = (info.Width - bitmapSketch.Width) / 2;
                float y = (info.Height - bitmapSketch.Height) / 2;
                existingCanvas.DrawBitmap(bitmapSketch, x,y);


                // redraw objects on non-visible surface 
                foreach (PathAndSpecs pathSpecs in completedPathsWithSpecs)
                {
                    existingCanvas.DrawPath(pathSpecs.path, pathSpecs.paint);
                }

                foreach (PathAndSpecs pathSpecs in inProgressPathsWithSpecs.Values)
                {
                    existingCanvas.DrawPath(pathSpecs.path, pathSpecs.paint);
                }


                existingCanvas.Flush();

            }

        }
        else
        {
            using (var canvas = surface.Canvas)
            {

                canvas.Clear(SKColors.White);

                // redraw objects on noni ble surface 
                foreach (PathAndSpecs pathSpecs in completedPathsWithSpecs)
                {
                    canvas.DrawPath(pathSpecs.path, pathSpecs.paint);
                }

                foreach (PathAndSpecs pathSpecs in inProgressPathsWithSpecs.Values)
                {
                    canvas.DrawPath(pathSpecs.path, pathSpecs.paint);
                }

                canvas.Flush();
            }


        }

        canvasView.InvalidateSurface();

        // take a snapshot of the surface and return as an image
        var snap = surface.Snapshot();

        // encode the image as a certain file type or convert to byte array for storage
        SKData jpgImage = snap.Encode(SKEncodedImageFormat.Jpeg, 100);

        // convert image to byte array
        byte[] jpgByteArray = jpgImage.ToArray();


        // save image as jpg to local database where it will be stored for upload.
        CreateAndSavePic(jpgByteArray, "Site Sketch");



    }




    private void CreateAndSavePic(byte[] dbPhoto, string documentType)
    {

        DocumentSet docset = App.Database.PhotoDocumentSetExists(sampleStationPassed.SampleStationLocalGuid, documentType);

        if (docset is null) // create new documentSet for pics
        {
            // create new document set locally and update properties
            docset = new DocumentSet()
            {
                DocumentSetLocalGUID = DependencyService.Get<IDeviceMethods>().GUIDGenerator(),
                DeviceInfoID = App.deviceInfoID,
                DocumentSetObjectLocalGUID = sampleStationPassed.SampleStationLocalGuid,
                DocumentType = documentType,
                DocumentSetObject = "SampleStation",
                DocumentSetObjectID = sampleStationPassed.SampleStationID
                //Description = "Site images for this station visit_" + System.DateTime.Now.ToString() + "_" + sampleStationView.StationName

            };

            // save new photo to local database
            App.Database.SaveNewDocumentSet(docset);

        }


        DocumentFile newPhoto = new DocumentFile
        {
            BinaryObject = dbPhoto,

            // should be new one every time
            DocumentFileLocalGUID = DependencyService.Get<IDeviceMethods>().GUIDGenerator(),
            FileName = System.DateTime.Now.ToString() + "_Image",
            FileStorageType = "SQL Binary",
            FileType = "Portable Network Graphic",
            // DeviceInfoID = Convert.ToInt32(App.deviceInfoID),
            DocumentSetID = docset.DocumentSetID,
            DocumentSetLocalGUID = docset.DocumentSetLocalGUID

        };

        // save new photo to local database
        App.Database.SaveNewPhoto(newPhoto);

    }