My understanding is this: Nightblood was created to imitate Shardblades and/or Honorblades. However, Shardblades are Splinters in physical form. I don't know much about Breath, but I don't think it's possible to make Breath coalesce into something tangible, so the Scholars had to use something already physical, and put the Splinters into it. The intricacies of Awakening meant they had to give the Breaths a Command, and Nightblood was "born". I don't personally think that Nightblood will give Szeth a Surge of any kind, mainly because his sentience does not come from Honor, Cultivation or a combination of the two.