I noticed that with another armor as well, can't remember which. You could use the enchanting/alchemy trick to boost your smithing up to a suitable level, and then improve it.
My smithing skill is at 100 but it will not let me craft the blades armor into Legendary ?? WHY ??
i think u need the advanced armor perk to improve the blades armor to legendary, might be wrong tho might be the steel armor perk. or it might just be a bug in the game. but i never have run into it i have multiple char with legendary blades armor. then again i run with lots of crafting mods.
Some unique armor and weapons require 100 skill and smithing gear bonuses and a potion to improve to legendary, dawnbreaker for example. I guess the blades armor is the same.
You can't because it's not tied to any smithing perk.
It's roughly equivalent to Orcish in base values, with the cuirass having a fair bit more than Orcish. However... Orcish armor has a perk to amplify smithing while Blades armor does not.
I had it turned into legendary once, I have enchanted fortify smithing in my gears and drank a fortify smithing potions and it went legendary and almost had the value of a daedric armor.