This summarizes how to create a full-screen fade-to-black effect in UEFN, like the kind you often see in games alongside the words “Now Loading.”
https://dev.epicgames.com/community/learning/tutorials/4dLE/fortnite-uefn-fade-to-black-across-the-entire-screen-with-no-gaps-regardless-of-resolution-sequencer-hud-message-device