Fat beef slices will turn black when cooked, and this can't be eaten. Generally speaking, its color is bright and gorgeous. This is normal. When the beef slices are cooked, the color will become lighter and there is no blood on them. The beef slices are not black after cooking. You should judge for yourself. Not all beef slices are good varieties. When the beef is cooked for too long, it oxidizes outside and loses water, so it turns black. Beef should be washed with blood, or it will turn black when cooked. Moreover, beef contains a lot of iron, and iron will be oxidized to ferrous ions during cooking, which will turn gray-black. Deteriorated beef will have a slight fishy smell, and it will have a serious sour taste as the meat rots. Deteriorated beef muscle is dark in color and fat is green.