Android : Android FrameLayout foreground setAlpha not working

I found this most excellent solution to the exact issue I am trying to solve here:

Blur or dim background when Android PopupWindow active

However, I cannot get the parent view to dim by setting the Alpha. I am using a rectangle shape as the solution suggests The Drawable is a GradientDrawable. Here's the code I'm using:

                @Override
                public void onViewPhoto(PostObject item) {
                    rootView.getForeground().setAlpha(220);
                    rootView.invalidate();
                    PhotoPopup pp = new PhotoPopup(getActivity(), rootView, item.getObject_Id());
                    pp.show();
                    rootView.getForeground().setAlpha(0);
                    rootView.invalidate();
                }

I've tried this on both KitKat (Asus Memo Pad) and JellyBean (Samsung Galaxy Tab 2) Any ideas?

For completeness: Here's the Layout

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/streamList_rootFrame"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:foreground="@drawable/shape_dim_window" >

    <ListView
        android:id="@+id/streamList_listview"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
    </ListView>

</FrameLayout>

And the shape itself:

<shape
     xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" >
    <solid android:color="#000000" />
</shape>