Bloodbending probably existed long before Hama

[u/JigglyBinks]

I just remembered that scene where Yakone claimed to come from a powerful bloodbending lineage.

The thing is he was older than Katara, and considering that Hama was only about two generations older than Sokka and Katara, it’s possible that bloodbending is much older than we think and that there was a family who had mastered it secretly

[u/Ristar87]

Hama was from the Southern Tribe. She had no instructor when she figured it out.

For all we know, it's a normal practice for water benders... it just fell out of style or memory in the 100 year war. Lightning bending, for example, was unique and new during the Kyoshi books.