In the Options, under Video, Visual, switch Darkness to 50 and increase the Gamma (mine is at 1.35). Adjust the gamma during day (outside) in the game, to make it bright enough to see farther, but not so much to cause eye strain.
'Brightness' sets the black point (how dark black is), which can be done exactly. 'Contrast' adjusts the overall luminosity of the screen (how bright white is), which should not effect the black point. A site I bookmarked is no longer there, but I found it cached at Google (minus the pictures, though);
Accurate Image Manipulation for Desktop PublishingIs your monitor still under warranty? If you set the black point properly, but can not increase the Contrast enough to display white as white, you should consider getting it repaired. Check it on a different computer / video card, if possible.