Black blood usually connotes an upper GI bleed. Maroon stool is usually caused by a brisk upper GI bleed. Bright red blood often connotes a lower GI bleed. Lower GI bleeds can be caused by diverticulosis, ateriovenous malformation, lower GI ulcer, ulcerative colitis, anal fissure, colon cancer, and hemorrhoids. If you are having chronic diarrhea or conversely severe constipation you can have bad internal hemorrhoids that will leave streaks of bloods on your toilet tissue. However, just mention this to your doctor, and if they are concerned they can work you up (colonoscopy + blood work).
Dr. S (spouse of Crohn's sufferer)