Corrected entry: If night-blood is passed parent to child, there's no way for them to exist as all night-bloods die in conclaves before child bearing age.
Plot hole: In s04e06 Clarke, Roan and Bellamy are chasing the Azgeda that stole the vehicle with the barrels, killing them all in the process. That leaves 3 drivers for 2 vehicles. S04e07 kicks off with both the rover and the yellow truck re-entering the camp. However, in later shots we see that both Roan and Clarke are at the island and only Bellamy drove home. If so, who drove the yellow truck back home since no other character returned from the island? (00:01:45)
Suggested correction: Different truck. The goal of the mission was to get the fuel (on the yellow truck) to Abby. Early in episode 7 as Clarke and Abby are talking, Clarke mentions that Roan is unloading the fuel. Meaning they still delivered the yellow truck to Abby as intended (minus the empty barrel). So it could not be back at the camp with Bellamy.
Correction: S5E1. Clark tells Madi: "Until I figure this out, I want you to hide in your secret spot. This is not up for discussion, Madi. The Flamekeeper scouts never found you there. Neither will they." This indicates that there were probably more nightbloods that never got found, therefore they could have passed it down to their children.
I don't think it acts like this, I think it's more so everyone has the nightblood gene, it just doesn't show in everyone - like the redhead gene.