I don't know about the programs you mentioned, but I'd guess you should be able to do what you can do in Sony Vegas Movie Studio. You add another video track that "overrides" your current video track. And you simply add in black screen to that track, during the segment where you want the black screen.

Another method is to cut out the video segment & add in black screen on the same video track. But I doubt you can simply select a section & color it black like you can in photo editing software.