A surprising number of references to Nightwing and his rogues' gallery have made their way into the Arrowverse considering that Dick Grayson has yet to make an appearance on Arrow or The Flash. Until recently, direct ties to anything relating to the Batman family of comics on The CW superhero shows were largely forbidden. This resulted in the writers of the Arrowverse leaning heavily upon the Nightwing properties which were related but not explicitly part of the Gotham City setting, in order to expand their shared universe while drawing upon the lore of the non-powered side of DC Comics' universe.
As the first person raised by Bruce Wayne to join him as a partner in crime-fighting as Robin, Dick Grayson quickly assumed a central role in the world of DC Comics. A founding member of the Teen Titans, Dick would take on the name of Nightwing in order to get out from under his mentor's shadow. He took the name from the legend of a Kryptonian folk hero once told to him by Superman, whom he came to admire and emulate almost as much as Batman in his dealings with the superhero community at large. This has led to Dick having a unique role as one of the more sociable figures among the next generation of heroes in the DC Universe while still maintaining a dark, mysterious facade as the protector of the city of Blüdhaven. Small wonder then that the Arrowverse should make mention of this, however indirectly.
Most of these Nightwing references amounted to simple Easter eggs referring to locations from the comics. Still, a few of the more notable villains from Nightwing's adventures also made appearances in the flesh, albeit in drastically different forms than in the comics.
In the DC Comics universe, Blüdhaven, NJ, is the one place in the United States more corrupt and lawless than Gotham City. Hard times fell upon the town when its two major products - whale meat and asbestos - were outlawed in the US. Legalized gambling saved the city just in time for organized crime to take it over.
What little has been seen of the Arrowverse version of Blüdhaven indicates that it's just as much a wretched hive of scum and villainy as its comic book counterpart. Blüdhaven generally makes an appearance or is mentioned whenever a location famed for its disreputable nature is required by the story. The exact location of Blüdhaven in the Arrowverse has never been established, but it's apparently close enough to Star City to have a direct train line running to it, as Oliver Queen threatened one man with being dropped onto the tracks in front of the 10:15 express to Blüdhaven.
Guillermo "Brutale" Barrera
The Arrow season 1 episode "Dead To Rights" opened with Oliver Queen intercepting an assassin named Guillermo Barrera as he arrived in Starling City by helicopter. In the Nightwing comics, Guillermo Barrera is the real name of the supervillain known as Brutale. Originally a high-ranking interrogator and torturer in the fictional Central American nation of Hasaragua, Barrera fled the country after a democratic revolution overthrew the government that employed him. Finding his way to America, Barrera put his knowledge of anatomy and expertise with bladed weapons to work as Brutale, wearing a horrific gargoyle costume covered with hundreds of knives of all sizes.
Walter Steele's Abduction In "The Undertaking"
In the Arrow season 1 episode "The Undertaking," Queen Enterprises CEO Walter Steele was abducted by a gang in the employ of Malcolm Merlyn. Steele was getting too close to uncovering Merlyn's master-plan to destroy the slums of Starling City with an earthquake generator and Merlyn needed to get him out of the way for a while. Luckily for Steele, his step-son Oliver Queen uncovered the kidnapping plot and was able to rescue him from the abandoned housing project in Blüdhaven where he was being held.
Roy Harper's Hideaway in"The Man Under The Hood"
When Roy Harper became infected with the rage-inducing Mirakuru serum in Arrow season 2, he broke up with his girlfriend, Thea Queen, and went into hiding outside of Star City. This didn't stop Deathstroke from tracking him down and trying to use Roy's blood as a means of mass-producing Mirakuru in order to empower his army of super-soldiers. It was revealed in the episode "The Man Under The Hood" that, when Deathstroke found Roy, he had been living in a homeless shelter in Blüdhaven.
Dudley "Torque" Soames
When Deathstroke and his army of Mirakuru-enhanced minions attacked Starling City in the Arrow season 2 finale, ARGUS Director Amanda Waller was content to destroy the city rather than chance Slade Wilson or any of his crazed super-soldiers surviving to fight another day. Thankfully, she was unable to order a drone strike thanks to the efforts of Lyla Michaels and John Diggle, who released Deadshot and the rest of Arrow's Suicide Squad members from their cells. One of these Suicide Squad recruits was a man named Dudley Soames.
Fans of the Nightwing comics will recognize Dudley Soames as a long-time nemesis of Nightwing. A dirty cop in the employ of the Blüdhaven Police Department, Soames was also on the payroll of the supervillain Blockbuster and pretended to ally himself with Nightwing shortly after he came to Blüdhaven. In truth, Soames attempted to play Blockbuster, the BPD and Nightwing against each other to his personal enrichment. Unfortunately, for Soames, he was not as indispensable or as good at hiding his treachery as he thought he was and Blockbuster twisted Soames' head around on his neck like a bottle-cap in response. Miraculously, Soames survived this, later establishing himself as a rival crime boss called Torque.
The New Brother Blood and The Renegades
As the title implies, Arrow Season 2.5 was a comic book based in the Arrowverse, which depicted several events set between season 2 and season 3 of Arrow. The book was notable for having been written by Arrow showrunner Marc Guggenheim and showcasing several adventures that would have been too expensive for Arrow to depict on a TV series' budget. Chief among these was a Suicide Squad story that pitted the show's versions of Deadshot and Bronze Tiger against a terrorist leader based on Black Adam.
The first major story arc of Arrow Season 2.5 centered around a new Brother Blood, closer in tone to the character from the original New Teen Titans comics, attempting to reform Sebastian Blood's cult. This second Brother Blood was eventually unmasked as Clinton Hogue - Sebastian Blood's former bodyguard. The team chased Hogue back to his hometown of Blüdhaven, where he enlisted the help of a group of mercenaries known as the Renegades to help him capture Felicity Smoak.
One of the Renegades utilized a special suit of armor that allowed him to generate electrical shocks. Though not identified by name, the costume and modus operandi resembled that of a frequent Nightwing villain called the Electrocutioner. It's also worth mentioning that Dick Grayson briefly adopted the codename Renegade while attempting to infiltrate the Secret Society of Super-Villains.