Someone once told me, with respect to ink drawings, that you should not be afraid of large areas of black. I think the same holds true for pencil, and even color for that matter. Very dark tones add a lot to a drawing, and it's important to use the full value range to achieve a realistic effect. When I was younger, I did almost all of my drawing in pencil, and usually an HB exclusively. While I was technically good (proportions and all that), I never could get the depth of realism without dark shadows. As I experiment and learn, I have noticed that it's those really dark areas that make the drawing more realistic. The key is not to be afraid of them.
I used a lot of black in the snow leopard drawing. Not just in the background, but also on the cat itself. Dark shadows also helped lift the fur of the koala and give depth to the leaves. Observe the change in values of different objects under various lighting. You may notice that the shadow side is a lot darker than you thought.