If you continue to scratch, then that area can become black and thick. This is called lichenification. It is possible you had some itchy eruption which got lichenified. On the other hand, you could be having a skin disorder which is black to begin with, for example lichen planus, lichen amyloidosis etc. Sometimes clinical examination is not enough to arrive at a diagnosis. In such cases, a skin biopsy may have to be done.