The black dirt on the cat’s nails may be caused by the accumulation of soil. If the cat usually plays with flower pots, it is easy to form black dirt in the nails. This is normal and there is no need to worry too much. However, if the cat does not have the habit of playing with dirt, then it is possible that the cat has ear mites stuck on when scratching its ears. You need to check whether there are ear mites in the cat's ears. If so, help the cat to treat them in time.