You can certainly pick absolute black (0,0,0 RGB) as a color, or even do a negative "color". If you're not even able to get that far, then maybe there's some fundamental piece of info lacking, or a misunderstanding. First of all, have you read the Color Picker docs?
http://planetside.co.uk/wiki/index.php?title=Colour_PickerThe other thing of course is that everything in your scene, every color, is affected by atmosphere and lighting. So in bright lighting, a very dark surface could look lighter, or in the distance it could look very blue even though it's black (because it has little or no color of its own, so you just see the atmosphere color).
As usual with almost any question, it's helpful to post examples of the problems you're having (and ideally what you want to achieve). I do wish you shared your tests and other images more, because you have a lot of (generally quite good and interesting) questions, but sometimes it takes a while to really get to the heart of what you're aiming for due to lack of visuals. Images are less subjective than description, generally.
- Oshyan
