Passage: Originally written as a spec script by Michael Blake, it went unsold in the mid-1980s. However, Kevin Costner had starred in Blake's only previous film, Stacy's Knights (1983), and encouraged Blake in early 1986 to turn the Western screenplay into a novel to improve its chances of being produced. The novel was rejected by numerous publishers but finally published in paperback in 1988. The rights were purchased by Costner, with an eye on directing it.Actual production lasted for four months, from July 18 to November 23, 1989. Most of the movie was filmed on location in South Dakota, mainly on private ranches near Pierre and Rapid City, with a few scenes filmed in Wyoming. Specific locations included the Badlands National Park, the Black Hills, the Sage Creek Wilderness Area, and the Belle Fourche River area. The bison hunt scenes were filmed at the Triple U Buffalo Ranch outside Fort Pierre, South Dakota, as were the Fort Sedgewick scenes, the set being constructed on the property.

Question: Who wrote the original spec script?
Michael Blake