Doctors may call this blood in stool or rectal bleeding. Blood in poo is one of the symptoms the NHS has highlighted in its 'Be Clear on Cancer' campaigns, as it may be a sign of bowel cancer. Although people are unlikely to talk about it, it is relatively common, affecting around 1 in 10 people in the UK.
http://www.webmd.boots.com/digestive-disorders/blood-in-stool-causes-and-diagnosis
Sometimes, the blood may not even be visible on the stool, and this is termed occult gastrointestinal bleeding. By far, the most common causes of visible blood in the stools are diverticulitis and hemorrhoids. Other conditions and situations that can cause blood in the stools include: colon cancer.
https://www.verywell.com/what-does-seeing-blood-in-your-stools-mean-796823
See your doctor if you notice blood in your stool or changes in the color, consistency, or frequency of your bowel movements. Easily treatable conditions such as hemorrhoids may be the cause, but it is better to err on the side of caution and get the correct diagnosis.
https://www.verywell.com/what-does-seeing-blood-in-your-stools-mean-796823
In brief: Depends. Dark stools may be caused by digested blood from a stomach bleed. A test for fecal occult blood should answer this. Some foods like blueberries or meds like pepto-bismol can also darken stools. See your doc and share any symptoms you have so you can get a good workup. Dark stools may be caused by digested blood from a stomach bleed.
https://www.healthtap.com/user_questions/83714-whats-wrong-if-my-poop-is-black
Blood in the stool should be evaluated by a health care professional. Rectal bleeding (known medically as hematochezia) refers to passage of bright red blood from the anus, often mixed with stool and/or blood clots. Most rectal bleeding comes from the colon, rectum, or anus and can be associated with diarrhea.
http://www.medicinenet.com/rectal_bleeding/symptoms.htm
Sometimes, bleeding from the gastrointestinal tract can be too slow to cause either rectal bleeding or melena. In these patients, bleeding is occult (not visible to the naked eye). The blood is found only by testing the stool for blood (fecal occult blood testing) in the laboratory. Occult bleeding has many of the same causes as rectal bleeding and may result in the same symptoms as rectal bleeding.
http://www.medicinenet.com/rectal_bleeding/symptoms.htm
There are multiple reasons to have blood in your stool. It could be from something simple like hemorrhoids or an anal fissure. Colitis, which is inflammation of the colon, can cause rectal bleeding, as can colon polyps or colon cancer, to name just a few. Blood in the stool can be very serious and should prompt a visit to your doctor so they can determine if further evaluation is needed. There are multiple reasons to have blood in your stool.
https://www.healthtap.com/user_questions/67761-what-is-wrong-when-i-have-blood-in-my-stool
As blood in poo is one of bowel cancer's early symptoms, it is not something to ignore. Anticoagulant drugs ( blood thinners). A side-effect of these drugs, including warfarin or aspirin, is internal bleeding. Inflammatory bowel disease(IBD) including Crohn's disease and ulcerative colitis.
http://www.webmd.boots.com/digestive-disorders/blood-in-stool-causes-and-diagnosis
Anal fissure. This is a small and painful tear in the anal area. Blood from anal fissures is likely to be bright red. Pain and bleeding may continue after a bowel movement. Anal fistula. This is a channel that forms between the anal canal (back passage) and near the anus.
http://www.webmd.boots.com/digestive-disorders/blood-in-stool-causes-and-diagnosis
Blood in the stools can be bright red or dark, black, and/or tarry looking, and it always requires your doctor's attention, although in many cases it will have a benign cause. Though colon cancer isn't to blame in all cases (the cause could be something less serious), blood in stools can be a signal of the disease.
https://www.verywell.com/what-does-seeing-blood-in-your-stools-mean-796823
what is wrong when blood is in your stool
Seeing blood in your stool may be a sign of bowel cancer. When blood is in your stool, it may be a sign of bowel cancer.