If you are talking about doing this during gameplay, this seems like something that might be easier to do with widgets.
I recommend making a widget with a completely black border that fills the screen. Then you can make a widget animation to make its render opacity go to zero over time. And, make two events that you can call in the widget: one that plays the animation forward and the other that plays the animation backwards.
Then, all you would have to do is add to viewport and have the widget play the forwards one on begin play / play backwards one when you go to a new level (with a short delay of the fade time after it).