So I am playing the Dawnguard DLC when I pickpocketed your thrall, the blind dude. Dexon or something like that, anyway I pickpocket his Akaviri sword and I'd like to dual wield it with my blades sword, but I can't improve it. Am I doing something wrong? I am level 81, with all of the light armor side of the smithing unlocked...
It is marked as stolen, and when I go to improve weapons it doesn't even show up.

I pickpocketed it from him too. I don't think you can improve it. Maybe because it's a one of a kind item?