Board Thread:Spoilers/@comment-28118206-20160403030006/@comment-25777978-20170917213437

I have a theory about that:

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.

So this is my theory.