Which preposition to use with darkenings

of Occurrences 31%

But behind these simple words lie hid centuries of strife and endeavor, and limitless darkenings of human hope.

in Occurrences 1%

The sun is setting, the tufts of heather darkening in shadow as they walk home.

to Occurrences 1%

Sweeping round a wide bay, the land drawls nearer again, the far-away blue darkening to purple, and then to green and brown.

Which preposition to use with  darkenings