Black raisins retain a darker black color because they are often sun-dried or dried under extreme conditions. They are also made from a different variant of grapes called the Black Corinth grapes. The higher the temperature and the more direct the sunlight, the more darker the raisin.
Black raisins are known for their dark and caramelized flavor. They are less sweeter than the golden ones, but nutritionally both contain the same nutrient profile.