Gradient Background on Flutter AppBar

This should work flawlessly:

return Scaffold(
  appBar: AppBar(
    title: Text(widget.title),
    flexibleSpace: Container(
      decoration: const BoxDecoration(
        gradient: LinearGradient(
          begin: Alignment.topCenter,
          end: Alignment.bottomCenter,
          colors: <Color>[Colors.black, Colors.blue]),
      ),
    ),
  ),
);