[Flutter] Problem with blurring a part of the widget

I'm having a problem making a portion of a widget to be blurred (I'm using a stack).

return Scaffold(
body: Stack(
children: [

Padding(
//sth very nice
),
Positioned(
top: 0,
bottom: height*0.85,
right: 0,
left: 0,
child: BackdropFilter(
filter: ui.ImageFilter.blur(
sigmaX: 5, sigmaY: 5,
),
child: Container(color: Colors.transparent,)
),
)
],
),
);


What I want is a little blurred area at the top of my page, that covers the below widget (in this case, the content of the padding).

Actually, what I have is that the blurring effect covers all the screen and this is very interesting because in my stack I have a Positioned widget with a precise dimension. It seems like the blurring effect goes over the limits ...

I put the image of the entired blurred screen:

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