The way smithing works in terms of upgrading weapons and armor has always confused me a little. I have a Smithing level of 100 and I have the Steel Smithing and Arcane Blacksmith perks. Can I upgrade my Nightingale Armor to Legendary as long as my Smithing is at 100, or do I need to put more perks into Smithing to upgrade it?

No perk for nightingale armor, so you need to boost your smithing up to 168 (with enchated gear/potions) in order to upgrade your nightingale armor. They are upgraded with void salts I believe.

Thank you this is exactly what I was looking for

Oh and also I've heard that the level 46+ nightingale blade can't be improved at a grindstone, does anyone know if this is true?

I have a question regarding daedric weapons like dawnbreaker, mace of molag bal etc, how do I make them legendary? I have 100 smithing, perks in all the heavy armour and arcane blacksmith yet I can only upgrade them to flawless :/.

Isn't that just the amount needed to get a "legendary" tag on the side? You can boost your smithing more to get even higher stats.

There are some unique weapons (and certain types of standard gear) that you can't improve beyond a certain point without the Fortify Smithing buffs mentioned above. If you have potions or enchanted apparel that will temporarily boost your skill, it may be possible, but even at 100 with all the perks it isn't. Sometimes it's because the weapon isn't classified as part of any materials group to which a perk applies, meaning for instance that you don't get any benefits from having the Ebony perk even if Ebony is what you use to temper the weapon. According to UESP that's definitely the case with Dawnbreaker and it may be so for many of the Daedric artifact weapons. There are mods that address a lot of those issues but if you can't make use of them you're kinda stuck.

Nice little sword you got there!!