In The 100, where do all the Nightbloods come from? They fight to the death until there is only one alive and that one becomes the commander so where do new Night Bloods come from? And if they all die fighting each other, how do they have children?
They come from the Nightblood serum created by Becca. She injected herself and became the first Nightblood and all concurrent Nightbloods are created by being injected with the same serum.
To quote the Wiki post:
The serum created by Becca, which causes a person to become a Nightblood, is also referred to as Nightblood.
Also, Grounders don't fight to the death but can be carriers for the blood. To quote the Wiki post again:
The majority of Grounders have normal colored blood, but may be carriers for the dark blood since Nightblood children can be born to non-Nightblood parents.
Note: Never watched The 100. Please feel free to correct me in the comments