You don't need to change that line. It isn't an exposed feature.
You can do keyed encryption and decryption by editing the code by hand. The key can be updated on that line, or passed in during encryption and decryption.
None of this is currently exposed, so it requires hand coding. The line won't be used at all if you are using our interfaces to interact with the database without editing the resulting code by hand.
This code was added for a specific user in a premier support ticket who was trying to mimic the blowfish encryption created by their Drupal application. He was then able to create his own login page and keep his existing database and login protection he had on his existing site. The feature seemed like it was worth keeping the code around even though we didn't update our interfaces to support it directly. At some point in the future we might update our interfaces to allow direct access to this feature.