Blackjack

Blackjack, is a friendly black pegasus who assists Percy 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.

The Sea of Monsters
Blackjack first appears in The Sea of Monsters, where he escapes from Kronos' army ship where Luke had been keeping him prisoner on the Princess Andromeda. 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
Blackjack has a larger role in this book; he gave Percy a ride from Camp Half-Blood all the way to the Smithsonian Air and Space Museum in Washington DC 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, 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
In The Last Olympian, he 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
In The Lost Hero, Annabeth mentions that Blackjack was used to search for Percy.

The Son of Neptune
Blackjack was mentioned by Percy after he drank the gorgons blood.

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".
 * Blackjack gets his name from the popular card game 'blackjack' a.k.a. '21's', this may be a reference to the fact that whenever Percy rides Blackjack, it is a 'gamble' due to Zeus, being god of the sky, could, at any moment, blast Percy to death while he is flying (although pegasi are considered to be common ground due to horses being created by Poseidon).
 * 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. Possibly Percy riding Blackjack during The Titan's Curse or The Last Olympian.