Blue Squares

The Blue Squares is a color gang in Ikebukuro. Several years prior to the start of the story, they fought with the Yellow Scarves in a conflict orchestrated by Izaya Orihara that ultimately led to the downfall of both gangs. Their founder, Aoba Kuronuma, scrapes together the remnants of his gang several years later and hands the reigns over to the leader of the Dollars, Mikado Ryuugamine.

Aoba's Blue Squares

 * Aoba Kuronuma (founder)
 * Mikado Ryuugamine (leader)
 * Yatsufusa
 * Yoshikiri
 * Neko
 * Gin
 * Nitari brothers
 * Yasushi
 * Houjou
 * Neishin
 * Mitsukuri

Ran's Blue Squares

 * Ran Izumii (former leader)
 * Horada (leader within the Yellow Scarves)
 * Higa (Horada's second-in-command)
 * Kyouhei Kadota (defected)
 * Saburo Togusa (defected)
 * Walker Yumasaki (defected)
 * Erika Karisawa (defected)
 * Shimada (defected)

Background
Aoba Kuronuma started Blue Squares when he was in middle school but handed leadership over to his older brother Ran Izumii in order to use him as a scapegoat should things go south. Ran proved to be a poor choice, as he frequently made decisions without Aoba's permission, and even initiated a gang war with the Yellow Scarves. The resulting conflict with the Yellow Scarves, mainly the kidnapping of Saki Mikajima, defection from within the Blue Squares of Kyouhei Kadota's group, and police involvement led to Ran and several other senior members being arrested. This nearly destroyed the Blue Squares, much to Aoba's annoyance. Not long after, he received an online invitation to join the Dollars and accepted, seeing it as an opportunity to rebuild his gang. After Ran was sent to juvenile detention, remnants of his group led by Horada joined the Yellow Scarves with intent to slowly take over the rival gang from the inside.

Hollywood Arc
At one point at night, Aoba and his fellow Blue Squares members roam around Ikebukuro at night. Aoba allows his Blue Square members to threaten different people while claiming to be apart of Dollars, but Aoba himself stays a close distance away from them and observe other things happening in the city. He notice Mikado and Anri running together (who were running to witness the Headless Rider Celty pass by), and even heard a loud "You petty thief!!" coming from somewhere away from his area (which came from Shizuo dealing with Hollywood and Egor).

One day at school, Aoba introduces himself to Mikado at school and brings up the Dollars. Aoba asks Mikado if he's a member of the Dollars, but Mikado tries to question him on what he's talking about until the Dollars mailing list arrives on both of their phones and the truth is revealed that Mikado really is apart of the Dollars. The mailing list was sent from an unknown person named Wakaba Mark, but Aoba reveals that he is Wakaba, and the one who sent out that message. Aoba soon explains to Mikado that he knew Mikado was a part of the gang because of the first meeting that happened at the plaza in front of Tokyo Hand. Aoba tries to ask Mikado if he's one of the top brass in Dollars, but Mikado denies, and later ask Aoba to keep things under wrap about Dollars. Aoba agrees to do so, but ask Mikado the favor of being shown around Ikebukuro. Aoba's tour to be shown around by Mikado with Anri tagging along would later be ruined by Toramaru members causing trouble within Ikebukuro. During a Blue Squares meeting at the end of the chaos that happened the day Aoba was supposed to be shown around Ikebukuro, Aoba sat with his gang and discussed how Mikado seemed so interesting. He talks about even with the situation is dire, those surreal and comic book situations is what Mikado loves. It's the type of guy Mikado is, and that's the reason why he started the Dollars. As some of the members start to throw out different questions, Aoba begins to speak about the fight between Yellow Scarves and Dollars. How the fight was only just starting to heating up, but it would be lame to just let the fight end without both of them doing much damage to each other. Blue Squares goal is to start a new spark between them, but first they need to get rid of Izaya Orihara. After Aoba commanded everyone not to touch Izaya sisters because they both kissed him and he likes them, one member said he would kill Aoba, but called him "Izumii." Aoba tells him not to call him by his former name because it reminds him of his brother.

Toramaru Arc
Aoba and his Blue Squares members unexpectedly show up at Mikado's apartment and take Mikado to an abandoned warehouse. At the warehouse, Aoba mentions how Mikado tried asking questions on the Dollars website about the feud with people from Saitama. Aoba reveals that he has the details, but it was his crew doing. He explains that him and all the guys inside the warehouse, as members of Dollars assaulted the Saitama gang, Toramaru. That day Toramaru was chasing Kadota van and Celty, Aoba and the rest decided to set fire to a bunch of Toramaru's bikes, and even sent 20 of the members to the hospital. One of the members join in the explanation and says that Aoba himself decided to toss a Molotov cocktail into the parking lot there the Toramaru gang was hanging out. Aoba reveals to Mikado that him and his group goes by another name other than Dollars, and that name is Blue Squares. Mikado starts to question what is it that Aoba wants, and Aoba explains that he isn't asking Mikado to become the leader of Dollars because that would ruin the philosophy that Dollars hold, but to become the new leader of Blue Squares, and that Mikado would fit as the leader because he also holds a "special" spot within Dollars as the founder of it. While Mikado becomes surprised that Aoba and the others know about him being the founder, Aoba shows that Blue Squares themselves have a pretty decent information network of their own.

Aoba gets two of the members to close the warehouse doors and guard the doors to make sure Mikado can't leave. Another member decides to grab Mikado tightly to make sure he listen and doesn't follow Aoba as Aoba decides to leave to meet up with Anri who he already started texting to mess with Mikado. Before Aoba can leave, Celty cellphone starts to goes off and she reveals that she's been hiding outside and peeping through a window while listening to the conversation Blue Squares was having with Mikado. Soon, Toramaru members find the place everyone is currently at and starts to fight with Aoba and his gang. Before the two gangs can officially clash, Aoba yells out and command everyone to stop, then tells his leader Mikado to make a run for it as the rest of the Dollars members hold back the Toramaru guys. Revealing to the Toramaru guys that Mikado was the leader that they were looking for, Toramaru tries to attack Mikado but is attacked by Celty and other Blue Square members that continue pretending to be apart of Dollars.

More chaos continues until the very end where Mikado finally accepts to become the new leader of Blue Squares. Mikado signs a contract, but in Aoba's blood by stabbing Aoba in the hand with a ballpoint pen because Aoba threatened Anri earlier in front of him.

Durarara!! SH
Horada is released from prison and attempts to take over the Blue Squares, but stands down after Aoba reveals he is Ran's little brother.

Trivia

 * As the name Yellow Scarves is a reference to the Yellow Scarves Rebellion, it is possible that the name "Blue Squares" could be a reference to the "Blue Sky" referenced in the Rebellion's slogan. The slogan is "The Blue Sky has perished, the Yellow Sky will soon rise; in this year of Jia Zi, let there be prosperity in the world!" (蒼天已死，黃天當立. 歲在甲子，天下大吉. ) In the context of the original rebellion, the Blue Sky was a reference to the Han Dynasty and the Yellow Sky was the rebellion; however, it is entirely possible that Izaya would have seen creating the Blue Squares as a poetic opponent for the Yellow Scarves.
 * Aoba has been quoted saying that the name "Blue Squares" was originally thought up by a sickly character known as Yatsufusa and the origin being that they "were almost like 'sharks stranded in shallow shoals.' Each one of us had a small blue rectangular piece of territory. You guys are like sharks fighting desperately just to protect that little slice of territory you have."
 * As of the end of volume 6 of the light novels, Mikado Ryuugamine is the leader of Blue Squares. However, Aoba plans to overthrow him at some point, and re-take control of his gang as well as Mikado's gang, the Dollars. It is hinted that he may do the same plan he did with Izumii, with Mikado, just as Izaya was going to do (mimicking the Kida and Saki incident).
 * Aoba, the Blue Squares' founder, has "Ao" in his name which means "blue" in Japanese while Kida, Yellow Scarves' founder has "Ki" in his name, which means "yellow."