Saw this on the first page of きつねのはなし by 森見登美彦, searched online but only saw this book as reference. Wondering how exactly would you read it...Thanks a lot.
Though 薄暗い is an everyday word and 常闇 is an established word, there is no word that reads 常暗い, at least as a common word in Japanese language. Neither 広辞苑 nor 大辞林 at hand carries 常暗 as a noun as well as 常暗い as an adjective.
If you found it, it must be a coinage of the author or weird resurrection of a long-ago dead word at best.
In my lifespun of 83 years living in Japan, I've never heard that word. I'm sure of that nobody would blame you and me for not knowing how to read it, what it means, and whether it really exists as a lgitimate word on the earth.
I have never seen this word and it seems to be an occasional or creative word usage by the writer. I'd read とこぐらい or とこくらい that would mean "ever-shady".
(morpheme) mainly attached to nouns or adjectives to indicate "unchanging" or "permanent".
For reference, the 新潮文庫 edition of きつねのはなし doesn't have furigana on this word, which suggests it neither has peculiar reading nor is compound ateji. Most probably is either of ジョウ, つね, とこ listed in 常用漢字表.