Find your WillowEngine.ini file inside the Documents folder, usually located here:
Documents\My Games\Borderlands 2\WillowGame\Config
To permamently disable the dark outline find and replace this line (under [Engine.Engine]):
DefaultPostProcessName=WillowEngineMaterials.WillowScenePostProcess
with this one:
DefaultPostProcessName=WillowEngineMaterials.RyanScenePostProcess
Final result
+ No more dark outline
+ No more vignette effect (slightly darker corners)
Screenshots comparison