Diverticular bleeding of the colon is very difficult to locate with endoscopic examination. In case of severe bleeding, too blood limits the procedure, in case of stop bleeding there isn't nothing to see. A contrast CT scan in emergency setting should be considered
I do not think that diverticular disease is the only cause of lower gastrointestinal bleeding. In emergancy settings a CT scan may be indicated only after a proctologic examination does not find other source of bleeding (haemorroids, fissures, rectal or anal polyops, solitary ulcer syndrome). Anyway, a successive colonoscopy is mandatory expecially in old patients.