Do you have ebony smithing unlocked? “The Nightingale Blade can be upgraded with an ebony ingot with the Arcane Blacksmith perk. However, it does not benefit from any Smithing perks. This means that the weapon cannot be improved past flawless quality without boosting the Smithing skill over 100. This can be achieved by using enchanted apparel or blacksmith’s potions to fortify Smithing.”

https://www.reddit.com/r/skyrim/comments/vc04oa/nightingale_blade/