The black mark could be a sharpie to remind him of something. The deformed fingernail next to it is the result of having slammed or pinched it in the past, as the nailbed is making a full nail now over the scar. I had one like it from hitting it with a hammer. This can be repaired by using a thick acrylic nail over it as a splint, the nail has to completely grow out about three times under the splint.