Just watched the bloodbending episode - Legend of Aang, S3E9, The Puppetmaster - again. We've never seen anyone other than Katara or Hama use bloodbending (and according to Hama, it's only doable under a full moon), so it seems reasonable to assume only Katara knows how to do it. I wonder if one reason for keeping Katara around - remembering she's only been in one episode so far - was to have her teach the art to Korra, but then I wonder what sort of circumstances would be necessary for her to contemplate doing so. Also, Hama, Katara and Korra are all from the Southern Water Tribe, so if Katara was to pass it on to Korra, it would become part of their heritage, which is what Hama implied she wanted when she said "I never thought I'd meet another Southern Waterbender. I'd like to teach you what I know so you can carry on the Southern tradition when I'm gone."
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We are living our lives
Abound with so much information