Blackjack

Blackjack is a friendly black pegasus that assists Percy Jackson when he needs transportation and can also communicate with Percy in his mind. Blackjack and other pegasi often go to get Percy to help them with their, or other creatures' problems.

History
"Gotcha, boss. Man, I hate seeing that boat"

- Blackjack in the Last Olympian

Blackjack was once a prisoner in Kronos' army ship, Princess Andromeda, until Percy released him. Percy mistook him as a female in The Sea of Monsters. Blackjack promised assistance when in need as a gratitude for letting him escape. When Blackjack flew to the ship again for Percy and Beckendorf's mission in The Last Olympian, Percy figured that Blackjack would rather have his mane braided like "My Little Pony" than be back there again.

The Sea of Monsters
Blackjack manages to escape from Kronos' army ship where Luke had been keeping him prisoner. After Percy saves him from an uncertain death, Blackjack devotes his eternal assistance to him. Blackjack, then and currently, resides in the pegasi stables in Camp Half-Blood. He is introduced as a mare in this book.

The Titan's Curse
At camp, before Percy goes off on his quest, Blackjack wakes Percy up, telling him that there is a little sea friend that needs his help. Later on, he gives Percy a ride from Camp Half-Blood all the way to the Smithsonian Air and Space Museum in Washington D.C. trying to catch up with Thalia, Grover, Bianca di Angelo, and Zoë Nightshade. He leaves Percy there and flies back due to his exhaustion of constant flying for such a long distance. Later, Blackjack and his friends Guido and Porkpie appear to carry Percy, Annabeth, and Thalia from a plane field near Mount Tamalpais to Olympus in order to attend the Winter Solstice meeting. Leaving them on a cheerful note, he asks Percy if he could use Percy's cabin as his stable if Percy does not come back alive. At Percy's incredulous expression, he quickly answers that it was "just a thought," before apologizing.

The Battle of the Labyrinth
Blackjack speaks to Percy at the beginning of the novel about the Labyrinth. He warns Percy about the dangers of the underground maze. Due to Percy's call, Blackjack, along with other pegasi, comes and carries Percy and his friends back to Camp Half-Blood after escaping the Labyrinth.

The Last Olympian
Blackjack lands on Paul Blofis' car while Percy was out with Rachel Elizabeth dare. He had Charles Beckendorf with him. Beckendorf and Percy, after an awkward farewell to Rachel, was carried by Blackjack to the Princess Andromeda, where he leaves on Percy's command. During The Battle of Manhattan, he carries Percy and Annabeth as reinforcements to the Apollo cabin who were defending the Williamsburg Bridge against Kronos' army. When Annabeth suffers a serious injury, Blackjack manages to carry her back to the camp's headquarters at the Plaza Hotel opposite Central Park. When the Clazmonian Sow appeared, Blackjack and Percy pursued it, and with the help of some Lion automatons made by Daedalus, they managed to defeat it. Finally Blackjack was forced by Rachel to bring her to Camp Half-Blood so she could become the Oracle, much to the irritation of Percy.

The Lost Hero
Annabeth mentions that Blackjack is being used to search for Percy.

The Son of Neptune
On a plane to Anchorage, Alaska, Percy remembers when he'd flown before. Though the details were a bit hazy, Percy remembers Blackjack specifically as well as the other times he'd been on a plane.

Personality
Blackjack has a highly informal personality - although he is willing to serve Percy, he calls Percy "boss" rather than "Lord", as well as once calling Dionysus "the wine dude". Blackjack has two best friends, Guido and Porkpie, who are also pegasi.

Trivia

 * Percy mentions that he doesn't let anyone ride Blackjack, except himself (but, obviously, Rachel didn't care what Percy thought and she must have thought her needs were more important, and Beckendorf had permission from Percy).
 * Ever since Percy indirectly freed him, he thought of Percy as a hero. So much so, that he woke Percy up to help any creatures that he couldn't help himself (like the Ophiotaurus caught in fishing net problem).
 * He appears on the cover of The Titan's Curse being interrupted by Dionysus' grape vines on top of the Chrysler Building and on the cover of The Last Olympian, with Percy wielding Riptide. As such, Blackjack is the only character so far (aside from Percy, himself) to appear on more than one book cover.
 * Blackjack only addresses Percy as "Boss", which Percy finds annoying.
 * His gender is somewhat inconsistent as in The Sea of Monsters he is referred to as a mare and a "she," but from The Titan's Curse onward, Percy refers to him as "he." The problem was asked by a fan at a Rick Riordan book talk. He replied "Either Percy's mixed up, or it's a mistake by the author, which, of course, could never happen."
 * Perseus, in ancient Greek mythology is also known to have been given a pegasus by the goddess Athena. Whether this creature was black or not is unknown but he has been portrayed as such in several cases.
 * In The Kane Chronicles, The Throne of Fire, Carter thinks he sees a boy on a flying horse. It could possibly Percy riding Blackjack during The Titan's Curse or The Last Olympian.