There's a slight color bleed and blur on the sides of the 4:3 "screen" as well, which I thought looked nice — plus having the effect fill the screen keeps the all the blacks the same shade, which I thought was less jarring than pure black on the sides.
Not saying it's necessarily a good choice, but that's what I went with in this one. Might do pure black in the future tbh. 🤷♂️