Darkness is not modable directly in the game, only by changing the current time of day.
Day has a darkness of 0 (where 0=bright as day and 1=pitch black) and it goes up to 0.85 at night, but this value is hardcoded into the game so you can never go above 0.85.

See also these topics:
Adjusting ...

Hey, Oke_oku here.

I was just wondering if someone could throw together a mod where night is actually pitch black. Ive done a little coding but not in lua, so i wouldnt know where to start, but i know that it could be really simple, or really difficult, depending on how the devs did the lighting ...

Im making a mod where it adds boxes that can be transported on belts,

At the moment im getting a error on startup saying 'Error in assignID, 'woodenBox' was not recognized id of entity'.
I think the problem is that i haven't made a entity for the box, but i cant find where the entity data is for ...

Does this mean that we get LAN multiplayer in a week???