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.
The 100 (2014)
1 corrected entry in season 3
Starring: Eliza Taylor, Paige Turco, Marie Avgeropoulos, Thomas McDonell
Revealing mistake: At the river to cross over to Mt Weather, the vine used to swing across the river is just a standard rope, camouflaged. You can see it best when Finn recovers it after Jasper has jumped.
Conner: Tell me again.
Raven: You're the most beautiful broom in a broom closet of brooms.
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.