I haven't found any clues. But in the search, I was led to this fascinating interview with A&M engineer Steve Katz, who worked on Hejira, Don Juan's Reckless Daughter, and Mingus.
https://jonimitchell.com/library/view.cfm?id=4622
OMG, my daughter was married at the Henson Studios. I know exactly where Blue was recorded! I was there all weekend and had no clue. Thank you for this post!!!