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"The blade is not affected by any smithing perks. PC Only This issue has been addressed by the Unofficial Skyrim Special Edition Patch; it causes the weapon to benefit from the Daedric Smithing perk." UESP
How do I replicate this feature in USSEP using Creation Kit?
In general, if an item is temperable, but does not benefit from a smithing perk, this is usually a result of the item lacking an appropriate material keyword. Find the Weapon record for any Daedric weapon (should have one of these FormIDs), look in its keyword list for a keyword indicating that it is a Daedric weapon (I would guess that the name would be WeaponMaterialDaedric, or something along those lines), and then add the same keyword to (each leveled version of) the Nightingale Blade. You probably also want to create a tempering recipe for NightingaleBlade05; these are under Constructible Objects, and you can just copy and modify the one for NightingaleBlade04.
(Hint: To quickly find the relevant COBJ record, right click NightingaleBlade04 in the Weapons list and choose "Use Info" - this will pop up a window of everything in the game that references that record, including its tempering recipe.)
