Oral melanin pigmentation: This is the most common cause of pigmentation on the tongue, often due to the accumulation of melanin. It can be physiological or a response to various stimuli, including smoking, medications, or systemic diseases.

Smoker's melanosis: This condition is directly related to smoking and can cause pigmentation on various parts of the oral mucosa, including the back of the tongue.
Hairy tongue: Although more commonly associated with elongation of papillae, some forms of hairy tongue can present with pigmentation, especially if there's an accumulation of debris or bacteria.
Oral lichen planus: While typically presenting with white lesions, some forms of oral lichen planus can have pigmented variants, which might be seen at the back of the tongue.

Oral melanoma: Although rare, melanoma can present as pigmented lesions in the oral cavity, including the tongue. Early detection is crucial for prognosis.
Kaposi's sarcoma: In immunocompromised patients, Kaposi's sarcoma can manifest as pigmented lesions in the oral cavity, including the back of the tongue.
Peutz-Jeghers syndrome: This genetic disorder can cause mucocutaneous melanin deposits, including in the oral cavity, and is associated with an increased risk of certain cancers.

Addison's disease: This rare endocrine disorder can cause generalized hyperpigmentation, including in the oral cavity, due to increased ACTH levels.
Nevus: Benign melanocytic nevi can occur in the oral cavity, including on the tongue, and present as pigmented lesions.
Malignant melanotic macule: A rare, potentially malignant lesion that can occur in the oral cavity, presenting as a pigmented macule.