How can I replace the right-most 30 pixels of my movie with black?
There is a free video conversion tool available on Mac, Windows and Linux called Handbrake. Handbrake does a lot of things, one of which is crop a video.

If you open a video and go to the picture tab you will get something that looks like this.



At top right you have Cropping. Select custom and then adjust the number of pixels on each edge you want removed.