Not sure if you ever solved your issue but this worked for me for with PixInsight v 1.8.8-4 (x64) Windows OS.
1. Download ffmpeg (latest version-64 bit-static) from
https://ffmpeg.zeranoe.com/builds/2. Extract zip file and place only ffmpeg.exe into ie: C:\Program Files\PixInsight\bin directory
3. XISF Image files MUST be resampled to a smaller size ie: 4656x3520 -> 1429->1080
4. Have blink do an auto stretch (or I prefer to permanently stretch all the linear images with ONE identical histogram stretch)
5. Load resampled images into Blink
6. Click Create Video in blink
7. Set arguments to the following: -y -r 25 -i Blink%05d.png Blink.avi
8. Set output directory to an empty folder
9. Run
The new folder will have the png frames and the AVI movie.
Hope this helps!
--Rich
PS:For short movies I have found that an animated GIF looks better than an AVI!