https://ayosec.github.io/ffmpeg-filters-docs/4.0/Filters/Video/blackframe.html
blackframe - FFmpeg 4.0.6 / Filters / Video
ffmpegfiltersvideo
https://ffmpeg.org/doxygen/trunk/vf__blackframe_8c.html
FFmpeg: libavfilter/vf_blackframe.c File Reference
ffmpegvfcfile