Hi, thanks for writing to HCM.
The hard tissue that you are appreciating over the injured area could be a SCAB under which the normal epithelium of skin grows. Its a normal finding.
Any soft tissue
devoid (without) of blood supply will turn black in colour and is said to be GANGRENOUS. One more important point is that dead tissue will not have any pain or touch sensation. Simply prick that area (which you doubt as dead tissue) with a needle, if you feel pain then that part is "viable"(alive), if not its dead tissue.
Hope this information is helpful. Good day