Seborrheic Keratosis: A common, benign skin growth that can appear as a dark rash, especially in older adults. The location on the lower back is consistent with this diagnosis, and the age of the patient increases the likelihood.

Eczema (Dermatitis): Can cause a dark rash due to post-inflammatory hyperpigmentation, especially if the patient has been scratching the area. The lower back is a common location for eczema.
Psoriasis: Although it typically presents with well-defined, scaly plaques, psoriasis can sometimes appear as a dark rash, particularly if the patient has a darker skin tone or if the lesions are in the resolving phase.
Tinea Corporis (Ringworm): A fungal infection that can cause a dark, circular rash on the skin, including the lower back. It's more common in warmer climates and can be contagious.

Malignant Melanoma: Although less likely, it's crucial to consider melanoma due to its high mortality rate if left untreated. A new, dark rash, especially with irregular borders, color variation, or rapid growth, warrants a thorough examination and possibly a biopsy.
Basal Cell Carcinoma or Squamous Cell Carcinoma: These skin cancers can present as a new growth or rash and are more common in older adults. Early detection is key to effective treatment.

Dermatomyositis: An autoimmune condition that can cause a distinctive rash, including on the back, but it's much rarer and typically accompanied by muscle weakness.
Erythema Multiforme: A skin condition characterized by target lesions, which can appear as a dark rash. It's often associated with infections or medication reactions.
Mycosis Fungoides (Cutaneous T-Cell Lymphoma): A rare form of skin cancer that can present with a variety of skin lesions, including patches, plaques, or tumors, and can mimic other skin conditions.