I had a PC make the Last Breath move tonight. He rolled an 8 so he got to make a bargain with the goddess, and he lived. He asked, "what are my hit points now?" We decided on 1, but I'm not sure that's right. How many hit points do you have if you survive your Last Breath?
0 HP, but stable
See page 26, “Death”:
Depending on the outcome of the Last Breath the character may become stable. A stable character stays at 0 HP but is alive and unconscious. If they receive healing they regain consciousness and may return to battle or seek safety. If a stable character takes damage again they draw their Last Breath once more and return to face Death.
So the PC would appear to everyone else to be “oh no, dying!” but they'll still be alive whenever their companions end up checking on them, and be revived by them. (Nobody knows that the PC visited the Black Gate, until they tell them! And then it's still a question whether anyone believes them…)
I wouldn't retcon your situation, though — just keep it in mind for next time.