Dark shadows in diamonds occur due to the stone having been cut deeper, which means it gets less light now. The deeper parts end up looking dull and lifeless because of this. Some stones have a dark shadow in the shape of a bow tie, but mostly, you just get jet black.
The darkness does not always have to be from a bad cut. A highly graded stone can have this defect, and usually, that is because of an inappropriate cutting style having been used. Most fancy cuts go deep, such as in the case of the marquise. Most step cuts show darkness as well. Some rough stones simply do not take well to these cuts, which is why deciding rightly at the beginning is essential.