Aye, I feel like whatever Callie's gender may be, it's complicated by the fact that they're an alien with a VERY different biology and culture, while also living among and fascinated by trolls and humans. I get the impression cherubs are biological hermaphrodites throughout their lifespan, with male and female personalities superimposed over the top. Callie and Caliborn switch control of their body with only superficial changes like the cheeks and tongue, which could just be chromatophores. As adults, cherub mating is described as quite similar to snails, which are hermaphrodites: the winner of the mating duel impregnates the loser. (Yes, snails do blackrom!) The same happens with cherubs, even if the loser is male.

....and Calliope doesn't show much attachment to traditional cherub gender or romance. They are in a long-term non-blackrom relationship with a human, for a start! Gender is already far more mental than physical in their species, and now she's negotiating all kinds of stuff with her past baggage and the culture they're currently living in. I do also get strong "she/they" vibes off how Callie is written, so I tend to go with that, but I struggle to find an exact human analogy for her gender.