So, if we suppose that Spren can be stabbed in the cognitive real because they have a more "physical form", then if nightblood hit a spren in shadesmar said spren would die, but if nightblood fought against a shardbearer with a living spren(kaladin, for example) in the physical real, said spren would "only" be hurt, or maybe broken(kind of like when the KR oaths are broken). Don't know if I'm getting that correctly, but I suppose that nightblood could act as a hemallurgic spike(because he feeds on Investiture), and so he probably needs a "true" physical contact to "spike" the opponent.