I’m trying to synthesize fisheye images from panoramas captured by SceneCaptureCube. After I solved the seams issues in Scene Capture Cube has seams in it? - #4 by CiaoTan, I came across this “blurred dark edges” problem, too.
Fortunately, the hint of @MostHost_LA is right. After I set the vignetting intensity to 0, the blurred dark edges disappeared.
So, Jason, instead of deleting the PostProcessVolume, you may try to enable it but set its vignette intensity to 0 (I wonder if Unreal Engine just set vignette intensity to a non-zero default value if you just delete it).