In order to unlock black hats and outfits you need to complete all Helmgart missions on legendary. Instead of having to open Okris tome every time I thought of a more elegant solution. When you open the mission selection screen it colours each mission icon with a metallic border based on the difficulty completed. However this is account wide, so once you do everything on legend the board will be solid gold.
My suggestion would be to change the border colour based on what character you have selected. So if you pick a mission as Sienna and you have never done War Camp on legend the border of that mission would be grey/bronze/silver instead of gold. This way you can tell at a glance which missions you still have to do.