Blackjack

Blackjack is a friendly black pegasus who was saved by Percy from Luke in the The Sea of Monsters, on the Princess Andromeda. Blackjack is featured along with Percy on the cover of The Last Olympian, and The Titan's Curse. In The Titan's Curse, Aphrodite said she was the one who sent Blackjack to help Percy from Camp Half-Blood. 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. They assist him with Percy's transporation in most books, including the Titan's Curse where Percy, Annabeth, and Thalia need to get to Olympus in time for the Olympian council.

The Sea of Monsters
Blackjack most likely first appears in The Sea of Monsters, where he escapes from Kronos's army ship where Luke had been keeping him prisoiner 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.

It's uncertain whether Blackjack appeared in The Sea of Monsters at all, but there is evidence that he might be the mare that Luke had brought out before his duel with Percy. However, he was referred to as a mare, which is a female horse, but he is referred to as a 'he' in the later books. So whether Blackjack was the mare or another pegasus in the background that had managed to escape Princess Andromeda during the confusion is uncertain.

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 buddies appears again carrying Percy, Annabeth and Thalia from a plane field near Mount Tamalpais (where Atlas carries his burden and also where the Mountain of Despair lies) 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. Percy stares at him, and he quickly answers, Just a thought!

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, saying "You ain't gonn be crazy enough to go in no maze, boss. Are ya? You'll end up in the glue factory!" Later on, coming due to Percy's call, Blackjack (along with other pegasai) 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 (interrupting Percy and Rachel Elizabeth Dare in an awkward moment, much to Percy's relief), with Charles Beckendorf on his back. Percy gets on and Blackjack carries them to the Princess Andromeda, where he leaves on Percy's command, though hesitantly, stating, Man, I hate that boat! During The Battle of Manhattan he carries Percy and Annabeth as reinforcements to the Apollo cabin defending Williamsburg Bridge against Kronos and his monsters. When Annabeth suffers a serious injury he quickly swoops down and managed to carry her back to the camp's headquarters at a hotel opposite Central Park (much to Kronos's annoyance, and Kronos swore to make pegasus soup). When the Clazmonian Sow appeared, Blacjack and Percy went in pursuit, and with the help of some statues, (that were really automatons made by Daedalus) they managed to defeat it. Finally he was forced by Rachel to bring her to Camp Half-Blood so she could become the Oracle.

Trivia

 * 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 refernce to the fact that whenever Percy rides Blackjack is a 'gamble' due to Zeus being god of the sky could at any moment blast Percy while he is flying (although Pegasi are considered to be common ground due to horses being created by Poseidon).
 * Of all of the books in the Olympian package, Blackjack is seen on two, more than anyone besides Percy.