There are many reasons for black finger joints, such as trauma, sun exposure, allergies, pigmentation, chronic diseases and other reasons. 1. Trauma: joints are subjected to external pressure, bumps, bruises, etc., which may cause subcutaneous capillary rupture resulting in bruising, thus leading to black finger joints. 2. Allergy: Skin allergic diseases, such as dermatitis, drug rash, may cause black finger joints. 3. Hyperpigmentation: Skin diseases caused by eczema, infections and other diseases may lead to hyperpigmentation after recovery, resulting in black knuckles. 4. Chronic diseases: chronic diabetes, for example, may cause black finger joints. Finger joints black, there may be other reasons, need to go to the hospital in time to check, clear cause.