One thing I remembered that is similar to your question about using the white dragonborn for dialogue. If you leave the dark urge’s name as The Dark Urge there are a few lines where the narrator will say “You say your name, The Dark Urge, out loud” instead of “You say your name out loud” if you have a custom name. The first instance is when you awake on the beach after the intro scene. There might be more but I can’t remember off the top of my head.
One thing I remembered that is similar to your question about using the white dragonborn for dialogue. If you leave the dark urge’s name as The Dark Urge there are a few lines where the narrator will say “You say your name, The Dark Urge, out loud” instead of “You say your name out loud” if you have a custom name. The first instance is when you awake on the beach after the intro scene. There might be more but I can’t remember off the top of my head.