FFmpeg overlaying and blending videos
I found a helpful post on stackoverflow that addresses this issue: https://stackoverflow.com/a/21400416/377875

Apparently, it's a problem with color space. Something like this works:

ffmpeg -i "$1" -i "$2" \
-filter_complex "[1:0] setsar=sar=1,format=rgba [1sared]; [0:0]format=rgba [0rgbd]; [0rgbd][1sared]blend=all_mode='addition':repeatlast=1:all_opacity=1,format=yuva422p10le" \
-c:v libx264 -preset slow -tune film -crf 19 \
-c:a aac -strict -2 -ac 2 -b:a 256k \
-pix_fmt yuv420p "$3"


(substitute $1 with your actual video, $2 with your overlay, and $3 with the output file name)