If you are cooking in your cast iron pan and you fin black residue on your food, do not get alarmed. This black residue is carbonized food. It is the reason why despite looking clean, your food is sticking to the pan. To get rid of the black residue, boil water in your pan and use a wooden spoon to scrape off the bits. Hot water helps in the process. After scrapping all the residue, wash the pan and season it again.