Dark Fairy

A Dark Fairy is an evil fairy whose powers have been corrupted, allowing for her to utilize dark magic.

Overview
A Dark Fairy is either a fairy who has turned to the side of evil, or whose powers become fueled by darkness like a witch's. A Dark Fairy may be a fairy who was corrupted by her negative emotions and uses evil magic rather than good magic fueled by light, or someone who was turned into a Dark Fairy through dark magic, like with how Lord Darkar and the Wizards of the Black Circle turned Bloom into Dark Bloom and Mitzi and her friends into Dark Fairies respectively. Just like witches, the bad nature of Dark Fairies' powers are shown by their dark clothing.

Normal fairies who do bad things are not necessarily Dark Fairies, as some Alfea fairies have displayed bad behavior and manners, and characters such as Diaspro have maintained normal fairy status, despite all of her misdeeds. Additionally, the Major Fairies of Earth in Season 4 have acted more antagonistically during their Age of Vengeance while remaining as normal fairies.

Dark Bloom
The first time a Dark Fairy was shown was in the Season 2 episode "The Spy in the Shadows," when Bloom was turned into an evil fairy by Professor Avalon's imposter at Alfea. At first, the change was subtle, as Bloom retained her regular Winx appearance as she stormed into the Secret Archives of Alfea and attacked Concorda to find Alfea's piece of the Codex, which she ended by giving to Lord Darkar once she successfully found it. Additionally, this spell seemed quite weak as Lockette's magic kiss was able to turn Bloom back to her old self for some time before becoming evil again.

The false Avalon then "healed" Bloom, who could no longer remember anything about what happened. Bloom's state was apparently caused by a kind of Shadow Virus which contaminated her. According to Headmistress Faragonda, only the person who had put Bloom under influence of that virus could heal her as easily as Avalon did, and this was partly how his identity as an imposter was revealed.

Later, Bloom was captured by Lord Darkar and turned into Dark Bloom, drastically altering her fairy form. Though her appearance and personality changed, her powers remained unaffected and were not fueled by darkness since Lord Darkar needed the power of light within Bloom's Dragon Fire to balance his darkness to gain access to the Ultimate Power of the Relix Dimension. This instance of the spell was also stronger than the one originally put on Bloom by Avalon's imposter, as Lockette's magic kiss had no effect. While Darkar himself said that Bloom had now passed over to the dark side for good, Dark Bloom was not a complete dark fairy as Bloom later snaps back to her true self when Sky admits his love for her. Bloom could also remember everything from when she became Dark Bloom.

In the Nickelodeon Specials, Professor Avalon's imposter was Lord Darkar in disguise, and he released an aura of darkness that turned Bloom into Dark Bloom upon contact.

Mitzi, Sally and Darma
Dark Fairies also appeared in Season 4 in the episode "Magic Lessons," when the Wizards of the Black Circle turned Mitzi and her friends, Sally and Darma, into evil fairies in an attempt to ruin the good reputation the Winx were gaining. Their dark magic and nasty attitudes caused negative feelings to appear in the hearts of nearby people in opposition to Believix, which causes good feelings to appear in the hearts of people.

However, the Winx (Group) were able to weaken Mitzi, Sally and Darma's powers by helping the nearby people and using their Special Believix Powers to reinstall good feelings within the hearts of the people. Eventually, their powers were weakened until they completely lost them, turning back into normal humans with no memory of anything they did as Dark Fairies, as well as possibly from when they met the Wizards, though it could be that it was a spell by them so that they do not have problems.

Tinkerbell
Dark Fairies also make an appearance within the World of Winx spin-off series, particularly in the form of the tyrannical ruler of the World of Dreams and main antagonist for most of the series, Fairy Queen Tinkerbell.

Unlike in Winx Club where the only known instances of Dark Fairies were created through outside influences, Tinkerbell became a Dark Fairy through her own powers becoming corrupted by her intense feelings of anger, rejection and abandonment towards Peter Pan, the man she had fallen madly in love with, after he left the World of Dreams without warning, and Wendy Darling, the one responsible for his leave. Upon becoming a Dark Fairy, Tinkerbell's appearance became darker and harsher as her powers manifested in the form of shadowy beings who followed her every whim. Her corrupted powers were even strong enough to cause Neverland to fall into darkness, appearing as a desolate wasteland.

It is unknown how long Tinkerbell acted as a Dark Fairy but, at some point, she began having her subordinates kidnap particularly talented young individuals to siphon them of their talents to strengthen herself under the belief that she was doing so to "regain the dream she lost years ago."

Tinkerbell does not regain her original, pure fairy magic until the Season 2 finale, "Tinkerbell Is Back," where her newfound love for Matt restores it, allowing for her to regain her true form in the process.

Trivia

 * When comparing Dark Bloom's appearance with that of the clothes Mitzi and her friends wear, Dark Bloom's fairy outfit still retains its sparkles just like the other fairy outfits, whereas Dark Mitzi, Sally and Darma's outfits do not sparkle and look more like civilian clothing.
 * Currently, there is no real explanation for this, though it could be because Bloom had already been a full-fledge Winx fairy by the time she was corrupted, while Mitzi and her friends were still human girls with no dormant magic powers of their own. Instead, they simply used the magic bestowed onto them by the Wizards of the Black Circle.
 * Interestingly, Beth McGraw, a character exclusively featured in Issue 96 of the Winx Club Comic Series, may also count as a Dark Fairy.
 * Just like Mitzi and her friends, Beth is given dark magic to wreak havoc and spread enough negativity to cancel out the Believix powers of the Winx Club by the Trix.
 * Though she claims to be a fairy while using dark magic, Beth never assumes the appearance of a Dark Fairy and wreaks havoc in her civilian clothes.