Nightblood isn't a spren. Have you read Warbreaker? Nightblood is a sword created with Breath, Awakened and given the Command to destroy evil. My only question is how he got transferred from Vasher's possession to Szeth's.
I know that Nightblood is a sword, and technically genderless, but it's less offensive to think of Nightblood as with a gender, and based off the description of the voice, I'd say Nightblood's male