I am creating my first video for TeacherTube, on how to embed the clips in PMWiki. I would like to blur out or block a piece of information. I know I have seen this in screencasts before, either the addition of the black box or blurred area. In fact I have even done this myself at one time but for the life of me can not think how to do it now.

I am working on a Mac using Snapz Pro, OminDazzle and iMovie, but am open to other tools if they can do what I need.

Any suggestions?