Dan Cassidy was hired to handle the special effects on a movie called the Blue Devil. He created an outfit for the character of the Blue Devil. The Kevlar costume was not only strong but it was made to be flexible so it could handle rigorous stunts. It was powered by miniature servo motors to add strength.
During the shooting the Blue Devil suit performed perfectly. Unfortunately and coincidentally a demon named Nebiros interrupted the shooting of the film. While fighting the Blue Devil Nebiros, assumed he was a fellow demon. So when the fight starts going bad for Nebriros he zaps the Blue Devil with a blast intended to drain his occult energy.
When the battle is over Dan Cassidy finds that his suit is a part of him. He discovers that the suit is organic and part of his body. Dan Cassidy is very upset about what has happened to him but because of his powers he ends up being a reluctant hero. He tries many ways and uses the help of many people but he is never able to remove his suit.
The Blue Devil continues to be a hero for years, even joining the Justice League for a while. He would eventually be killed by the Mist along with a few other heroes, by holy water placed in the sprinkler system. Because the Blue Devil is part demon he is reduced to bare bones. He would later be resurrected by Faust, the son of longtime JLA adversary Felix Faust. Blue Devil would later work with the super hero team of magic users the Shadowpact. He left the team in place of Zauriel who attacked him (under orders) due to Blue Devil's popularity that was causing humans to make deals like his. Blue Devil left the team to attempt to deal with the problem.
The blast by Nebiros in their original battle, aside from fusing the suit to Dan, also changed the trident that was part of the costume. Prior to the battle it was a simple flame thrower with some pyrotechnics; after the battle it had become a powerful magic device linked to the Netherworld. When the gateway between Earth and the Netherworld is open it is at its most formidable making the bearer a match for a legion of demons. It is thought controlled and answers to the mind of either Dan Cassidy or Nebiros. If either holds it they are dominant lord; when not in physical contact it can become a battle of wills for control so far Dan has always been the victor.
During the Infinite Crisis, the Blue Devil was the bouncer for the Oblivion Bar. When Enchantress tried to ask people to join her fight against the Spectre, he comes and tells her it's suicide to do something like that. He goes with the rest of Shadowpact to fight Eclipso (then possessing the mentally-unbalanced Jean Loring). He tries to send her to hell with his trident but she makes the comment that she isn't from hell. He then is thrown blocks away by a back slap from Eclipso. He comes back and asks Enchantress what is she doing being in the same spot since the fight started. She tells him that she is tapping into the energy that Captain Marvel gets his power from and is giving her own powers to him. He then asks her if she could tap into everyone in the world with magic and so she does. He then grabs his trident and throws it into Eclipso which leaves her on the ground with dark energy leaking from her. Blue Devil punches Enchantress when she turns evil knocking her out.
Blue Devil appears in Reign in Hell, fighting for Satanus. While battling the Demon Etrigan, he accidentally frees Lobo. Later, Etrigan takes his humanity. When all the demons are turned into humans, he is as well. Etrigan then restores his demonic half, which had been switched with the human half when Blue Devil was transformed.
Blue Devil has been seen helping the JSA in the city of Monument Point. It is unknown if he will join the JSA officially while staying in the city. Blue Devil and Alan Scott are seen encased in stone behind the throne of the White Witch who has been possessed by one of the Legion's enemies. She now calls herself the the Black Witch. It is unknown at what point in history this occurred, but it is easy to deduce it was painful, they are seen in some amount of agony while being entombed into the 31st century.
Dan is operating as a crimefighter in downtown Los Angeles, using the Blue Devil suit, which he has to wax his chest to wear. He comes into conflict with Black Lightning, who believes that he is working for Tobias Whale, during one of his excursions. The fight is ended when authorities show up and he is next seen interceding in a car chase during which a tracker is planted on him. Tobias's men follow him to Graveyard Pictures studio, where Dan lives with his grandfather who is a writer, actor, director, and studio mogul. His grandfather warns him about the suit, saying that it is something dangerous and that nothing good will come from it, but Dan thinks that saving people is worth it. As he is removing the suit, Tobias's men kill his grandfather and he turns to attack them. Tapping into the demonic power of the suit, seemed to have killed one of them and dropped another off a building onto a car until Black Lightning managed to convince him to stop.
Several days later, he spoke at his grandfather's funeral where he met up with his high school friend and fellow football teammate, Jeff Pierce. Having recognized him as Black Lightning, he revealed his identity as Blue Devil and arranged to meet with him at Graveyard Studios, which his grandfather had left him, along with a letter that explained their family collected dangerous, mystical artifacts powered by people's belief in them. By making cheap movies where they were treated as a joke, he drained them of their power. After revealing this, Danny suggested that they team up to take down the Whale.
He made cameos as a member of the Justice League in 'Justice League Unlimited'. However, he only had one, brief speaking moment and otherwise was not an important character.
He later appeared as another background character in Young Justice. He fought one of the Injustice League's plants in Taipei with Flash and Captain Atom.