Blacking out specific pixels in MP4 without re-encoding Basically is it possible to set the value of specific areas of pixels to black without re-encoding an MP4 video completely in an editor or application (Premiere, Ffmpeg, etc).

I'm wondering if such a capability exist to work directly on the compressed data as opposed to having to decode and re-encode.
No, it is not possible.  Mpeg-4 compression is based around storing information about patterns found within the video.  It doesn't deal with particular pixels.  Blacking out certain pixels alters the pattern and thus alters the way the video would compressed, potentially significantly.