So a possible theory is that Nightblood may have been made with the 1000 breaths needed, but something about his awakening lets him suck the breaths from those he kills. Which may indicate a reason that he's black and why he's the most invested object in the Cosmere. Because he just keeps killing and getting the breaths.
Additionally, I wonder if you had a glove or something similar made out of the same material (aluminum) as the sheath, if Nightblood would affect the holder as he normally does.