Filter Blur + Mask

I know how to mask in Flash and also how  to make a video for Blur on AS3 ...

Filter Blur

var MyBlur: BlurFilter = new BlurFilter(8,8,3);

video.filters= [MyBlur];

Mask

var box:Sprite = new Sprite();
box.graphics.beginFill(0x000000);
box.graphics.drawEllipse(170, 70, 500, 500);
box.graphics.endFill();

video.mask=box;

Now how do the two together? Put a mask on a filter?
Detail, the  filter is applied in a video.

Example image q  did in Photoshop.