The director had a lot of creative freedom on this project; the film's backers essentially 'handed him a blank check'. [=let him direct the film exactly the way he wanted to]
At the end of the movie, the narrator goes back and 'fills in' (all) 'the blanks'. [=the narrator provides missing information about the movie's plot]
He says that the first hour after the accident is a complete 'blank'. [=he cannot remember anything that happened for an hour after the accident]
(..) Hey, Jacques... I'm not Jacques. I am the Great Went. I am the Muffin. And what a muffin you've got, eh? I am as blank as a fart. Chug-a-lug, Donna. Yeah. I haven't seen you since I was thrown out of the One Eyed Jack's. What else did we do together? (..)1992 Twin Peaks: Fire Walk with Me