Marcus Daniels was a laboratory assistant to Dr. Abner Croit, a physicist hoping to build a device capable of tapping into energies from other dimensions. Croit always looked down on Daniels’ inferior knowledge, making him feel useless. Daniels always wondered what it would be like to harness the energy they were researching, to be powerful. After an accident bathed him in the extradimensional energy of the Darkforce, he had the chance to find out. Calling himself Blackout, his body was now flushed with power, becoming a surface of control of the Darkforce dimension. However, despite his powers threatening to go out of control, he escaped from Croit’s attempts to cure him and fled.
Blackout returned to the laboratories, however, as he needed the stabilizer device to control his energies. But he also returned to find revenge on Croit-- Blackout’s sanity began to suffer as well. He believed Croit was researching energies from "Black Stars" and that his body now generated such energy. He further believed that Croit was defrauding the government with his research and had bribed a judge to frame him for stealing his secrets. Instead of an accident causing his powers, Daniels thought Dr. Croit willfully subjected him to it an experiment exchange for dropping charges against him.
Nova encountered Blackout on his way to exact revenge, and Blackout easily defeated the young hero. Before Nova could catch up with him, Blackout returned to the labs, killing Croit and his new assistant by letting them “merge with the color spectrum” (actually, by shunting them to the Darkforce dimension.) Following another skirmish with Nova, Blackout himself vanished into this dimension when he fell back onto the stabilizer during the battle, destroying it.
The stabilizer somehow ended up at Project: PEGASUS, the governmental energy research laboratory. An attack by the Lava Men and the presence of Captain Marvel (Monica Rambeau) managed to activate the device, which transported Blackout back to Earth. Still suffering from the delusion that Croit was still alive and out to harm him, he tried to escape. Blackout was coerced by another Project captive, Moonstone, into freeing herself and other superhuman criminals in order to gain revenge on their captors. The villains were confronted by the Avengers, but Blackout and Moonstone escaped by nearly causing the nuclear core of the complex to meltdown as a distraction.
Moonstone, a former psychiatrist, treated Blackout in order to learn his true origin, but he remained in an irrational state. When the Avengers tracked them down, Blackout opened up another aperture into the Darkforce dimension in attempting to escape them, sucking himself and Moonstone inside. Moonstone helped Blackout navigate through the dimension, eventually arriving on the surface of the moon. She hoped to find another stone similar to that which gave her powers, but they encountered the Inhumans and their ally, Dazzler, who defeated the villains and returned them to Project: PEGASUS on Earth.
Moonstone next appeared recruited by Baron Zemo to form his version of the Masters of Evil, and she brought Blackout in tow. By this time, Blackout's mental condition was so unstable that at times he sank into a nearly catatonic state, only responsive to Moonstone's manipulative commands. Zemo, however, had created a device to mentally force Blackout to obey his commands. Blackout was instrumental in Zemo's takeover of the Avengers' headquarters by sending the entire mansion into the Darkforce dimension in one of the team’s darkest hours. Doctor Druid, used his psychic ability to break through Zemo and Moonstone’s manipulations, restoring some of Blackout’s mental faculties. With relative sanity, Blackout resisted Zemo's mental commands, the strain of which made Blackout collapse from a massive brain hemorrhage.
Blackout’s body was remanded into the custody of the Commission on Superhuman Activities. Years later, his body was taken by Baron Zemo, manipulated like a puppet on strings to serve as a member of his team as he confronted the new Thunderbolts over the life of their member Photon. During the battle between the two teams, Zemo revealed Blackout was merely a shell in which he kept the Smuggler trapped. This had the desired effect of turning his brother, Atlas against his team, and Zemo gained the upper hand. He also used Blackout’s access to the Darkforce dimension in severing Photon’s physical form.