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You should be able to lift out some of the darks with a kneaded eraser. Don't scrub it, just press and lift, then remold the eraser and repeat. (If you don't remold between lifts, you just grind the same charcoal back into the paper.) You won't get it all, perhaps, especially if you smudged some of it in with your finger (which would impart an oily binder -- impossible to erase).
By the way, I'm not saying not to use your fingers. I do it in graphites, charcoals, and paintings. Just be aware that the oils are going to make the former two mediums difficult to remove.
Also, when you're using a toned paper, take advantage of the benefit of already having your midtones (or darks, depending on the paper's tones) taken care of for you -- you needn't repeat them. Your sand color should represent most of your midtones (halftones). Look for the darks and the lights (for which you might make judicious use of white chalk or Nupastel), and the form will "miraculously" come together.
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