Make a UMG Widget, put an image on it with the color black. Add the Widget to Viewport from your blueprint. Also, you can animate effects like fade to black, wipe or other things all on the Widget.
Here is documentation: widget docs
I’m trying to create a scenario, so when a player walks into a certain area, their screen is blacked out, its for the ending of a game, however, i’m not really sure how to go about doing it, as I don’t know of any commands for something like this.