Brennan: "This is the way they explained it to me. This guy, Marcus Daniels, a.k.a. Blackout, was shrouded in darkness, his soul, everything about him was shrouded in darkness until he comes across this Cello player. And the music was the only light that came into his soul, and it gave him hope and it gave him some sense of what light was. I thought to myself, oh my god, you don’t even need to explain anything else. Because music, to me, is what gets me through most of the time. My kids, my wife, and music. That aspect of it totally intrigued me. I thought that was quite beautiful and very interesting, that aspect of the character."
The cello player he refers to is probably Audrey, played by Joss Whedon favorite, Amy Acker. That means he'll certainly run afoul of Agent Phil Coulson, who was her long distance boyfriend before he was "killed".
In the comics, Blackout was a lab assistant who was bathed in the power of the Darkforce dimension, giving him near limitless dark cosmic energy. He's battled the Avengers on multiple occasions and was a subject in Project PEGASUS, the mysterious agency we saw in The Avengers movie as the keepers of the Tesseract.
