Whenever I begin a new material, or switch the object in the scene, it develops dark black patches that can only be removed when I go into Materials>Default>Edit>Normal, and uncheck "Normal", leaving "DirectX Normal" checked. However, as stated, if I change the object in the scene, it re-checks "Normal". Is there any way to permanently uncheck this box?
You should have that box checked if you are using a normal output. The symptoms you show though look similar to those you see if you plug a grey heightmap straight into the normal output rather than via a normal node. Can you confirm you have not done that?