GR p.4 "with blue shadows to seal its passage"

[Jonathan Post]

I'm attempting a very close rereading of GR and have already come upon a
puzzling knot that has stumped me: in the long sentence at the top of p. 4,
I'm not sure how to interpret "...maturing rust, developing through those
emptying days brilliant and deep, especially at dawn, with blue shadows to
seal its passage, to try to bring events to Absolute Zero ..."

--What is "developing"? the various smells or the rust?
--What does the "its" in "it's passage" refer to? the dawn? the development
of the rust?
--And so, what are the "blue shadows"?

-J
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