The stool color would be in varying shades of brown under normal condition. It is the bilirubin and decomposition process which gives the color to the stool. Several factors can cause change in stool color. Presence of blood in your stool is not a matter of concern if it is occasional and limited. It can be due to certain foods you have taken last night or due to hemorrhoids. However if there is persistent change in stool color you need to seek medical help.
Blood in the small intestine would be bright red in color since the iron molecules of the blood is not yet oxidized. As the blood travels through the intestine it undergoes oxidation process taking light red color or maroon color which is reflected in the stool. In medical terms bloody stool is described as melena or hematochezia. Any blood leakage with stool need not always be due to bleeding of internal organs. It is possible to have black tarry stool due to drinks or food items and even certain drugs.
Melena :
Melena can be described as leakage of blood in the upper portion of the abdomen like that of esophagus or stomach. Several kinds of gastric juices and enzymes act upon the fresh red blood changing its color to dark brown or black. This blood while passing through the rectum produces black tarry stool. Melena can also be caused due to blood leak in the intestine or even inside the colon.
Hematochezia :