In the "Sea of Monsters" ch. 18 "Party Ponies Invade" pg. 242 second paragraph states "Before he could say anything, Agrius, the bear-man, burst onto the deck leading a flying horse. It was the first pure-black pegasus I'd ever seen, with wings like a giant raven. The pegasus mare bucked and whinnied. I could understand HER thoughts. SHE was calling Agrius and luke some names so bad Chiron would've washed HER muzzle out with saddle soap."
Yet in all the other books Blackjack is clearly a guy.