Since FancyMenu v3.6.0, the mod has support for playing MP4 videos!
There is a Video element and a menu background type that lets you play videos.
There are also the following actions to control video backgrounds and elements:
And the following placeholders to get information about video backgrounds and elements:
To use the Video element and menu background type, you need to have the MCEF mod installed, which is used as backend for video support.
You can download MCEF from CurseForge¹ and Modrinth².
When you see only black instead of the video playing, it is most likely not related to FancyMenu, but MCEF. MCEF probably failed to load correctly, so FancyMenu can't use it.
In this case you should better reach out to the MCEF devs and ask them for help in their Discord: https://discord.gg/rNrh5kW8Ty
The same can happen with the Browser element when MCEF fails to load correctly.
¹ https://www.curseforge.com/minecraft/mc-mods/mcef
² https://modrinth.com/mod/mcef