He who is Paul said on Thu, 28 Sep 2017 04:26:07 -0400:

I could do the whole job using FFMPEG and netpbm suite
(if the area to be blurred is static), but I don't think
anyone wants to hear about it :-) Just search until you
find a modern method.

He who is pjp said on Thu, 28 Sep 2017 12:34:13 -0300:

I use VirtualDub, With the plugins it does pretty much anything I've
ever required. Note - not always the most intuitive to use but you can
always find a page on google that will guide you doing something you
don't know how.

What's interesting, which often happens with Freeware, is not only that the
suggested app doesn't even show up on the first two pages of Google hits
for the best freeware for the task, but that someone else (Pooh the Cat)
suggested VirtualDub, but Mayayana kindly tested it and it wouldn't even
open his MP4 file!

Of course, if it's just that VirtualDub can't handle MP4 files then that
can be fixed with the older versions of Super freeware (before they went to
the dark side) or the newer versions of Handbrake freeware on Windows, but
video-format conversions are almost never pretty affairs.

Looking at the VirtualDub web site for clues about handling MP4 files
http://virtualdub.org/
https://sourceforge.net/projects/virtualdub/

The first reference says outright "VirtualDub is mainly geared toward
processing AVI files", so, Mayayana's observations that VirtualDob didn't
handle MP4 files from Android screen recorders seem to be on the mark.

The good news is that this first reference does mention that it can "blur"
video, but it doesn't say whether it can do that within a selection area.
http://virtualdub.org/features.html

The second reference implies VirtualDub is mostly for "capture" and not so
much for editing of video but then it references the first site for more
information.

As we all know, the cost of freeware is finding the right one for the job.
The *easiest* and *best* way to find the right freeware for the job is to
find someone (usually an expert but not always) who has done the job with
freeware or with Payware (Payware is great to learn "how" it's done and
that it "can" be done).

From my cursory due diligence on VirtualDub, with conversion of MP4 to AVI,
VirtualDub might be able to "blur" selected areas of the screen - but, from
experience, I find conversion to have its own set of problems with almost
no gain so it would be best if we can solve the original question of:

Q: Has anyone on this ng ever "blurred out" selected sections of an MP4
using Windows Freeware (if so, what do you recommend)?