you should really pay attention when they're talking about nightbloods, it's very important!! i'm not sure if you understood what they talked about them in this episode and the previous ones, but i'll recap just incase; the first commander had nighblood (blackblood), and ever since then every nightblood that was found (no matter their clan) was brought into the capital to train and get ready for the conclave which happens when the last heda dies. the spirit of the dead heda will "choose" the next heda. this will all be explained way better, but to understand the logic you should really know the background info!! also ontari was never brought to the capital to train even though she should've been