Black outlines in Borderlands

Warning: If you make a mistake you may have to reinstall Borderlands. Back up Borderlands and any game files before continuing.

You are looking to disable the outline shader via the Borderlands WillowEngine.ini file under Documents\My Games\Borderlands\WillowGame\Config\. You can tweak many aspects of Borderlands by modifying this file. There is a tutorial on the Borderlands Wiki and I highly recommend you read it before trying anything.

You are looking to change the line:

DefaultPostProcessName=WillowEngineMaterials.WillowScenePostProcess

To this:

DefaultPostProcessName=WillowEngineMaterials.WillowScenePostProcess_cinematic

This will change your view from this:
_{(click on an image to view the higher-resolution, original image)}

To this:

It's a feature of the cel-shaded visual style of the game - you can't turn the outlines off.

An easy way to disable the black outlines (outline shader) is by using the Borderlands Config Tool / Configuration Editor. Download mirror sites of current latest version, v2.1.4: [1] [2]:

It backups the configuration files it edits, so you can revert its changes anytime.