Black lemon is a culinary name commonly used for dried lime, also called black lime or loomi. Despite the English name, the ingredient is generally lime. Its concentrated sourness, bitterness and citrus aroma season stews, rice and other dishes in Persian, Iraqi and Gulf cooking.
See the dried-lime entry for uses and naming. Check the product when a recipe simply says dried lemon: ordinary dried yellow-lemon slices are a different ingredient.
Reference: Further reading.