Think of your grayscale when working with black and white conté on toned paper... The color and value of your page corresponds to one of the grays toward the middle of the scale, ideally, middle gray. You can make the dark values (to the right of middle gray above) with your black and the light values (to the left of middle gray) with your white.

As working with value in any other way, the greater the range of values you create--with both the black and white--the more descriptive your drawing can be relative to light and form.