Q:

Put a black box image over existing video using ffmpeg

I am using blob tracker, but have a timecode on video files. I want to overlay a black box over it, therefore preventing the timecode interfering with the tracker.

A:

Going by the FFmpeg docs for overlay, you're going to want something like:
ffmpeg.exe -i in -vf "color=black:20x10 [over]; [in][over] overlay=5:5 [out]" out

Replacing 20x10 and 5:5 with the width x height of the box and the position of the timestamp as necessary.

