Diverticulitis: This condition is common in older adults and can cause blood diarrhea due to inflammation of the diverticula in the colon. The age of the patient and the duration of symptoms make this a plausible diagnosis.

Colorectal Cancer: Given the patient's age, colorectal cancer is a significant concern. Blood in the stool can be a symptom of cancer in the colon or rectum.
Inflammatory Bowel Disease (IBD): Conditions like ulcerative colitis or Crohn's disease can cause chronic blood diarrhea. Although they can occur at any age, the onset in older adults is less common but still possible.
Ischemic Colitis: This condition, caused by reduced blood flow to the colon, can lead to blood diarrhea and is more common in older adults.

Infectious Colitis: Certain bacterial (e.g., Salmonella, Shigella), viral, or parasitic infections can cause blood diarrhea. Although less likely in a month-long scenario, they are critical to rule out due to the potential for severe outcomes if not treated.
Angiodysplasia: Vascular malformations in the colon can cause recurrent, painless bleeding. This condition is more common in older adults and can be a source of chronic blood loss.
Intussusception: Although rare in adults, intussusception can cause blood diarrhea and is a medical emergency requiring prompt intervention.

Radiation Proctitis: If the patient has a history of radiation therapy to the pelvic area, radiation proctitis could be a cause of blood diarrhea.
Vasculitis: Certain types of vasculitis, like granulomatosis with polyangiitis, can affect the gastrointestinal tract and cause blood diarrhea, though this is uncommon.
Lymphoma or Other Lymphoproliferative Disorders: These can involve the gastrointestinal tract and cause a variety of symptoms, including blood diarrhea, but are less common causes.