Board Thread:Spoilers/@comment-28118206-20160403030006/@comment-46.103.80.13-20160403121153

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.

Again, it's a theory in progress!