I can't say what caused the cut - is she wild-caught? If so, it could just be a battle-scar or some injury from living out in the wilds. I've had a mantis develop a dark-ish scar (or scab, whatever you call it) over a cut, which is just a part of the natural healing process, I believe. To be on the safe side, just be sure to monitor your mantis closely, to make sure the black doesn't spread, and to see if she is eating and behaving normally.