While we're all a bit stunned by the fact that Yang actually PUNCHED Blake, it's a moment of "My fist forgives you". Just because Yang and the rest of Team RWBY welcomed Blake back into the team after the Battle of Haven doesn't mean that Yang's completely forgiven Blake for leaving her for two volumes. We as an audience and Blake should know by now that Yang has abandonment issues stemming from her biological mother Raven leaving her and her father when she was but a baby, her stepmother getting killed on a mission, and Taiyang becoming neglectful in his grief to the point that Yang had to be responsible for herself and Ruby when she was about 3-5 years old. Blake leaving the team after the fall of Beacon (in a well-intended but misguided attempt to protect her loved ones from Adam) and Ruby herself running away from home on what was (at the time) a suicide mission with what was left of Team JNPR really hurt Yang, but Yang figured there would be no way in hell that Qrow would leave Ruby to fend for herself, so she could, by finding her mother, track Ruby down almost instantly (and to be fair, Yang pushed Ruby away in her own grief at losing Blake). It was Blake's departure that felt like the most like a betrayal to Yang.



So yeah, I can literally see Yang punching Blake for leaving, but forgiving her afterwards.