Greetings,
I got the message that night was approaching, and indeed the lighting did dim as expected. However, once the "Night has begun" message appeared, the game promptly went back to proper daylight, and there was little evidence of increased native activity. The only mods I have installed that should affect biters themselves are Rampant and Natural Evolution Enemies. Attached are a log file and screenshot of the lack of darkness https://pastebin.com/V7BJrWjh https://imgur.com/a/BIvfxJO

I tried leaving a game running for a bit (with Day length: 60, Length of first day: 1 in the Mod Settings - Map), it goes and stays dark as expected.
It might have to do with CredoTimelapseMod, as it may change the brightness in order to take screenshots.
I never played PitchBlack in singleplayer, but if your pollution cloud is rather small and you got (un)lucky and don't have nests nearby there won't be any attacks.

Can you upload that save somewhere so i can take a look what is going on with the darkness?

It appears that it was CredoTimeLapseMod. I tried disabling it and loading the save again directly, but daytime persisted. I reverted to a previous autosave before nightfall without CTLM, and things stayed dark, so I think we're all good then. Attached save: https://drive.google.com/file/d/1a29xjolUFGkkJoDKO8EoJxiHazsmQadI/view