For anyone interested I have found a solution for Windows 10, It looks as though it isn't possible on Windows 8, like David Heffernan mentioned, DwmEnableBlurBehindWindow was removed from Windows 8, however Microsoft re-introduced a solution for achieving this effect in Windows 10.

• Windows 10: Solution using SetWindowCompositionAttribute
• Windows 8: No solution
• Windows 7: you can continue to use DwmEnableBlurBehindWindow

I hope I am not late to the party. You can use the SetWindowCompositionAttribute, but you are then forced to set the WindowStyle to "None" and implement your own native Window funtionality and handles. Also inheriting from a custom control is quite complicated. However, long story short. There's BlurryControls.

You can find it via NuGet by browsing for "BlurryControls" or check out the code yourself on GitHub. Eitherway, I hope this is helpful.

On GitHub you will also find a sample application.

Windows 8

As much as a dead end that'd better be completely forgotten, we might need to make sure our programs behave consistently across versions. If there is no need for a dynamic background and especially if the window is relatively small (a prime candidate would be a message box or similar over our own application window), the solution of capturing the screen behind the window and blurring it manually might work. In the Loaded event handler:

var screen = window.Owner.PointToScreen(new System.Windows.Point((window.Owner.ActualWidth - window.ActualWidth) / 2, (window.Owner.ActualHeight - window.ActualHeight) / 2));
var rect = new Rectangle((int)screen.X, (int)screen.Y, (int)window.ActualWidth, (int)window.ActualHeight);
var bitmap = new Bitmap(rect.Width, rect.Height);
using (var graphics = Graphics.FromImage(bitmap))
  graphics.CopyFromScreen(rect.Left, rect.Top, 0, 0, rect.Size);
border.Background = new ImageBrush(ApplyGaussianBlur(bitmap.ToBitmapImage())) {
  TileMode = TileMode.None,
  Stretch = Stretch.None,
  AlignmentX = AlignmentX.Center,
  AlignmentY = AlignmentY.Center,
};

where border is an outermost Border, normally left completely empty, only used on Windows 8.

The helper function is the usual bitmap conversion:

public static BitmapSource ToBitmapImage(this System.Drawing.Bitmap image) {
  var hBitmap = image.GetHbitmap();
  var bitmap = System.Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap(hBitmap, IntPtr.Zero, Int32Rect.Empty, BitmapSizeOptions.FromEmptyOptions());
  DeleteObject(hBitmap);
  return bitmap;
}

ApplyGaussianBlur() is best solved with a direct bitmap manipulation package like WriteableBitmapEx, with the necessary modifications.