iMovie Directions
Open iMove and import your video
Import your black square
Drag your movie into the timeline
Drag your square in the row above your video
Choose the square (double click it) and change its properties to be "picture in a picture".
Resize the black square (in the properties area) using the blue balls in the corners and overlap the area your wanting to censor
Change the duration of that black square to how ever long you need to have it on screen
As your subject moves around you may need to copy and paste and reposition the black square to keep up with the movement. Don't try animating the black square (you don't need it to be pretty). My suggestion is to make the black square big enough that it covers as much as possible given the timeline. That way you don't need to copy and paste as often.
