Each December in Hollywood, a list of the best unproduced screenplays of the year, as decided by studio executives and producers, gets passed around for a chance to shine.
In the past, many of the scripts were "small" concepts or "too weird/indie" to guarantee major box office, despite attaching name stars, so their inclusion on the list prompted interest in up-and-coming writers (like Diablo Cody at the time) and buzz about the power of the story on the page.
More recently, however, heavy-hitters like Aaron Sorkin are being highlighted. In my opinion, this kind of defeats the purpose of the Black List. I love Aaron Sorkin's work, but it's not like he's struggling to get produced. His script for "The Social Network" (about Facebook) may be technically unproduced as of this second, but it's being filmed (with Justin Timberlake) and will be released next year.
For more on the correlation between the Black List and eventual box office performance, check out this great article from the L.A. Times.