A Dark Fairy is either a Fairy who has turned to evil or whose powers become fueled by Evil and Darkness more like a Witch's powers while the powers of normal Fairies are fueled by Light. A Dark Fairy can also be a person who is able to become a fairy but instead of gaining good magic gains evil magic. It seems that an evil spell is needed to turn people into Dark Fairies, like Lord Darkar turned Bloom into Dark Bloom or when the Wizards of the Black Circle turned Mitzi and her friends into Dark Fairies. Just like Witches, the bad nature of Dark Fairies' powers are shown by their dark clothing.