Place the video on the timeline. Place a solid black generated media event on the track below it. Apply the cookie cutter effect to the video, choose the circle, cut away section, and set the size very small, maybe 0.100 to start. You can use keyframes to adjust the size and positioning of the circle from frame to frame as the subject moves around.
You can add the cookie cutter effect multiple times if you have more than one face to black out.
I know you asked about 'blacking out' but you could also use Kellys directions to 'blur' out by placing a copy of the track one video on track 2 and applying a blur effect to it instead of the soliod black media. You would do everything else the same.. Wpould take longer to render though.