I have a theory in progress. So, Becca is the reason Nightbloods exist in the first place. She created the black blood that allowed her to survive on Earth. So, those people had descendants and their descendants had even more descendants. I think that the blood is inherited in the family and only some people are actually born with it. I think it's like a genetic anomaly. Sometimes children don't look like their parents but like a distant relative or grear-grandparent. Same with the Natblida. The blood becomes that way when something triggers a genetic factor that allows the blood to take the properties of the black blood.
I thought Natblida is randomly born and when they are born, they are raised by present commander. When commander dies, those big enough to raise a weapon fight till death. When new commander is chosen, they start to gather Natblida again. That's probably why eldest Natblida in Lexa's court didn't look any older than 13. My only question is why was Lexa not raised by previous commander, like Aden, but by Anya.
Also, life of a commander seems pretty short. If a commander's reign lasted 30 years and there was 3-4 commanders only, there is absolutely no reason why anyone including Ontari could not recite names of all commanders.
Titus says he was Flamekeeper for the previous four Commanders before Lexa so it doesn't sound like they live very long lives, especially with Lexa's acceptingly fatalistic attitude. That means they'd need a refill on the Nightblood population every five to ten years would be my guess.
Either way, only 97 years have passed so it's not like they can have that many Nightbloods if they're all genetically related to Becca? Meaning Becca had kids? And wouldn't they be able to track her children to find future nightbloods (Iike Ontari)?
This whole genetic Nightblood thing sounds like made-up hogwash.
When Becca came back to earth she brought the night blood with her. The first people that came out from the tower of what was soon to be "Polis" were the first nightbloods. Titus even mentioned it to Murphy when he tried to explain how Becca fell from the Sky like they did. Not sure if Becca had kids but the most probable thing is that she injected the night blood into those people in Haz Mat suites and as they reproduced then the nightblood became hereditary. Also, I guess the point was for everyone to eventually become nightbloods but somehow these Maya acting people keep getting them killed in conclaves untill one remains to be the commander.
It´s probably that the black blood is a recessive gene, like the blonde hair or the blue eyes. So, for example, if your father is a nightblood and your mother is a redblood, you have a 75 percent of posibilities to be a redblood and 25 percent to be nightblood.
I'm assuming like all recessive traits the natblida blood isn't always visible in the carrier's phenotype. The same way that someone with blood type A can have a child with blood type O if both they and their partner carried the recessive 'i' allele. This way if someone whose ancestor was a natblida still carries this allele in their genotype, they may have a kid that's a natblida. Also, not all natblidas fight to become commander as it can be assumed not all families want their child to fight to the death.