You can’t simply use the same syntax to create a sprite as you might commonly see in AS3 or even on the Flex documentation. You have to include the UIComponent for it to actually show up.
Here’s the code:

import flash.display.Sprite;
import mx.core.UIComponent; // gotta have this

private function init():void {
	var square:Sprite = new Sprite;				// create the sprite
	var uiref:UIComponent = new UIComponent;	// create the uicomponent
	addChild(uiref);							// add the uicomponent to the stage
	uiref.addChild(square);						// add the sprite to the uicomponent
	square.graphics.beginFill(0x000000);		// customize your sprite
	square.graphics.drawRect(0, 0, 100, 100);
}

Without that UIComponent your sprite won’t show up in the stage.