Listener @M0t0rbreath also asked this question.
The problem with Blood Bending is that when it’s brought up in A:TLA it comes with some attachments: 1) it’s the most evil of the water bending techniques and 2) it can only be used during a full moon.
Blood Bending was developed by an imprisoned Water Bender like metal bending was developed by an imprisoned Toph, but whenever someone uses Blood Bending it’s always mentioned how…I dunno, “generally evil” it is.
I think it would be weird for Korra to have to embrace a darkness in her to win. She may ask someone (and if she does, please let it be Gran Gran Katara) to teach her, but I’m guessing someone will tell her that evil power is the shelter of the weak, and she’s the Avatar, so she needs to suck it up. : )
-Da7e-