The Black Coat (黒のコート Kuro no kōto?) is a magical form of clothing that allows the person that wears it to safely pass through the Corridors of Darkness. It is most noticeable as the uniform for Organization XIII, but chronologically is first used by Master Xehanort in Kingdom Hearts Birth by Sleep as an alternative to the Keyblade wielders' traditional armor. Several characters outside of the Organization wear this coat, including Riku, King Mickey, Ansem the Wise, and a Moogle.
The Black Coat's main purpose is to resist the corruption inflicted upon one's heart when they pass through a Corridor of Darkness. It also seems to be able to change shape to fit its wearer, as seen when Xion shifts from her normal form to Sora's, and when Riku changes into his Ansem form.
Somehow, when Riku and King Mickey cast their coats off, the black gloves, pants, and boots worn under the coat disappear. Furthermore, any trace of the wearer's clothing under the coat disappears, such as Riku's collar, jeans, and sneakers in his second attire, or Mickey's trademark white gloves.
All these hint that donning the coat transforms whatever clothing the coat's bearer is wearing under it into the black gloves, pants, and boots seen on all the coat's wearers. However, Roxas's sneakers are visible under his coat in several of his appearances.
The Black Coats were originally designed for Roxas and Riku in the secret video Another Side, Another Story... simply as a way to disguise them while they were still being designed for Kingdom Hearts II.
Nomura stated in an interview that his initial idea for "The Gathering" was to have Terra, Aqua, and Ventus wear similar coats to the Organization XIII coats to hide their at-the-time undesigned faces, but felt that he could not use black coats once again, and so he stuck with armors instead.
The black coat is a full-length black coat with a hood and a waist-high slit going up the back (similar to a duster). It has a large silver zipper that fastens at the top and zips down to seal the coat. It has silver drawstrings for the hood decorated with a large silver bead hanging from the end and a silver chain ornament adorned with four large cylindrical silver beads that fastens to a loop on either side of the collarbone region.
Some coats have more angular shoulder padding while others have different hemming or sleeves with a slightly different shape or length (as in the case of Axel, whose shoulder padding is more angular and sleeves are more close fitting than those of a standard black coat).
About fourteen years before Kingdom Hearts, Master Xehanort uses a Black Coat to travel to the Destiny Islands and leave his former apprentice, Ventus, whose heart was left broken by the creation of Vanitas. However, Xehanort doesn't seem to need any protection for the Corridors of Darkness, since he himself will admit in one of his reports that he no longer needs his armor.
Four years later, while Xehanort first tries to piece together the χ-blade with the hearts of Vanitas and Ventus, and even takes possession of Terra's body, Young Xehanort arrives at this time, with the goal of recruiting Vanitas and Terra-Xehanort among the real Organization XIII. He eventually meets Terra, Ventus, and Aqua in the Land of Departure, and even goes to face them one by one, to finally return to his time of origin.
When Riku and King Mickey leave Castle Oblivion after the elimination of Vexen, Larxene, Lexaeus, Marluxia, Zexion, and Ansem, Seeker of Darkness, "DiZ", actually Ansem the Wise, offers them Black Coats so they can become undetectable to the rest of the Organization and use the Corridors of Darkness safely.
Terra-Xehanort, as an apprentice of Ansem the Wise, led several more or less dangerous experiments on the human heart, but after he had banished Ansem the Wise to the Realm of Darkness and officially usurped his identity, these experiences led him and the other Ansem apprentices to divide themselves into Heartless and Nobodies. Since then, the Nobodies of Ansem's apprentices, led by Xemnas, Xehanort's Nobody, founded the first Organization XIII and made the Black Coat the uniform of their members. Even the Moogles wore a Black Coat after establishing their trade at the Castle That Never Was.
Riku and King Mickey wear their Black Coats at their various confrontations with the members of Organization XIII, eventually being named "Impostors" by the Organization, resulting in the Organization attempting to eliminate Riku several times. After Riku's capture of Roxas, DiZ and Naminé temporarily modify his memory before hiding it in the Virtual Twilight Town, making Roxas the first member of Organization XIII not to wear his Black Coat after receiving it.
King Mickey finally withdrew his Black Coat during the Battle of the 1000 Heartless, after Goofy rescued him from a rockslide and "certain death," while Riku continues to carry him until the explosion of the Kingdom Hearts Encoder, after which Ansem the Wise finally purged Riku's heart of the presence of Ansem, Seeker of Darkness, and restores Riku to his original appearance. In the final confrontation between Sora, Riku, and Xemnas, the latter replaces his Black Coat with No Heart's armor, then with a Silver Coat covered with stylized black marks. The Black Coat is also worn by the Mushroom XIII, the Absent Silhouettes, and the Organization XIII Replica Data.
In order to determine the source of a mysterious message that appeared in the first Jiminy's Journal, King Mickey decides to digitize it. During this process, the collective memory of the Journal settles in Data-Riku, who wears a Black Coat, which resulted in Data-Riku being confused with Organization XIII. Data-Roxas also appears, with the aim of "protecting" Data-Sora from his own pain.
After her fight against Terra-Xehanort, Aqua is sentenced to wander for several years in the Realm of Darkness to stop at the Dark Margin, where she finds Ansem the Wise, himself returned to the Realm of Darkness after the explosion of his Kingdom Hearts Encoder, and wearing a Black Coat.
Following the fall of Organization XIII, all the former apprentices of Ansem the Wise end up being reconstituted and regaining consciousness in the Radiant Garden, but only Isa and Lea keep their Black Coats among them. Meanwhile, Young Xehanort arrives from the past with several other incarnations of Xehanort to gather the real Organization XIII. As they interfere with Sora and Riku's Mark of Mastery exam, each of the members wear a Black Coat. In the final stages of their plot, they place Sora under the influence of a nightmare in order to prepare him to become their thirteenth vessel of their new Organization. When Riku first tries to free Sora, the nightmare is manifested as the Anti Black Coat, who wears a Black Coat tinged with purple hues at its extremities.
The real Organization XIII continue to wear their Black Coats during their missions, until their final confrontation with the Seven Guardians of Light. Only Master Xehanort doesn't wear a Black Coat, while Vanitas, Terra-Xehanort, Ansem, Seeker of Darkness, and Dark Riku eventually remove theirs during their respective battles. Lea, Roxas and Xion are the only Guardians of Light to keep their Black Coats until Xehanort's ultimate defeat.
In Re Mind, it's revealed Xehanort traveled back in time when he was younger and met the Master of Masters, who gave him a Black Coat. This explains the coat's origins: it was created by the Master of Masters and passed on to both Luxu and Xehanort, the latter of whom would, in turn, pass it onto the members of Organization XIII and leading to it being replicated by Ansem the Wise.