You could do this in kdenlive if you "create title clip" and draw a rectangle over it...
Here's what you would do:
1) load the video into kdenlive and add it to the timeline (preferably the second video timeline so you can put something over top of it)
2) "add title clip" and select the option to "show background" in the title clip you're editing.
3) draw a rectangle using the rectangle drawing tool in kdenlive over the area that you want to cover up.
If the rectangle needs to move, you may need to animate the title card. For long videos with extended periods of rectangle-blocking, you may need to create a lot of title cards. For more information, you can visit
kdenlive.org. They may be better able to help you in their specific forums.
EDIT: Also, if you have the effects installed, you can apply effects to the clip itself (might be easier than doing title cards). When you have the clip in the timeline selected, click "effect list" and go to "blur and hide" -- there's a few options that may be of use there.