If you find black specs in your food, that usually means the seasoning is breaking down and needs to be removed. The easiest way to completely remove seasoning from cast iron is to place your piece in a self-cleaning oven on the cleaning cycle. When the oven is done cleaning, your cast iron will be completely stripped of its seasoning. Be extremely careful removing any cast iron from the oven after using this method. It will be VERY hot. It's best to leave the cast iron in the oven to cool until it can be safely handled and
re-seasoned.
If you don't have a self-cleaning oven, you can use a propane burner to remove the seasoning. Turn your Dutch oven upside down on the burner over medium heat until all the seasoning has burned off. If you have a heavy seasoning, more heat and time are probably necessary to completely remove the seasoning.