I don't have The Text in front of me, but surely you recognize "boneless blue" as Annie Proulx's phrasing? In this context I've always taken "boneless" to mean "cloudless," though I can't exactly say where I picked up that notion and it may predate my reading of Annie Proulx.
I know, for example, that a ship moving fast enough to create a white wave at the bow is said to have "a bone in her teeth," so I may be projecting from "bone" = "white wave" to "boneless blue (sky)" = cloudless, clear blue sky.
If I remember correctly, Annie's use occurs in her narration of the last horsepacking trip in May 1983.
Hi Jeff! Yep, you're right that it's about their May 1983 trip. In my copy of Close Range, the phrase "boneless blue" appears on p. 274. And, based on Proulx's context here, the phrase does seem to reference a cloudless sky. This phrase comes early in the important sentence about Jack feeling like he could drown looking up into the sky.
"Ennis, weather-eyed, looked west for the heated cumulus that might come up on such a day but the boneless blue was so deep, said Jack, that he might drown looking up."
And, Ellle!
Glad you're feeling better Bud! Chickenpox really are miserable.