as3 how to set a transparent hole in opaque MovieClip

Hi,

I'm trying to create a dark overlay layer on top of some graphics, but want to "cut" a transparent circular hole in that dark layer so only that circular part will be seen, and the rest will seem dark (similar to method used to explain a game or an app in Android, which hides most of the screen by a dark layer, and reveals only parts I want to highlight). I've been trying to use BlendModes, specifically the ALPHA or ERASE modes, but this is not working for me...

This is what I have so far:

I created a black rectangle of the size of the stage and gave it the LAYER bland mode (as i read was needed for the ALPHA blend mode to work), then I added my circle and gave it ALPHA blend mode and set alpha = 0 to be transparent. I see the black layer, but not the transparent circle inside it...

Any help would be appreciated!

var mc:MovieClip = new MovieClip();

var rectangle:Shape = new Shape; // initializing the variable named rectangle

rectangle.graphics.beginFill(0x000000); // choosing the colour for the fill, here it is red

rectangle.graphics.drawRect(0, 0, stage.stageWidth,stage.stageHeight); // (x spacing, y spacing, width, height)

rectangle.graphics.endFill(); // not always needed but I like to put it in to end the fill

rectangle.blendMode = BlendMode.LAYER;

rectangle.alpha = 0.8;

mc.addChild(rectangle); // adds the rectangle to the stage

var circle:Shape = new Shape(); // The instance name circle is created

circle.graphics.beginFill(0xFFFFFF, 1); // Fill the circle with the color 990000

circle.graphics.lineStyle(2, 0x000000); // Give the ellipse a black, 2 pixels thick line

circle.graphics.drawCircle(stage.stageWidth/2, stage.stageHeight/2, 200); // Draw the circle, assigning it a x position, y position, raidius.

circle.graphics.endFill(); // End the filling of the circle

circle.blendMode = BlendMode.ALPHA;

circle.alpha = 0;

mc.addChild(circle); // Add a child

stage.addChild(mc); // Add a child