Was there ever a black Sith Lord in Star Wars canon or "Legends"?
To be clear I mean black like:
As far as I can tell, no. The two closest would be:
Both examples are from Legends. Although Sora Bulq does appear in TCW and AOTC, he is only a Jedi in those media.
It is worth noting that the species that Mace Windu and Kar Vastor came from, the Korun, are all force-sensitive, and of those described have all been depicted as black (except for the character Chalk, as Hypnosifl mentions below, whose skin is described ambiguously as 'light-skinned' possibly meaning relative to all the other Korun, as in 'lighter' skinned). Windu had darkness within himself but he channeled it with Vaapad, the seventh lightsaber form. Both of his contemporary practitioners of Vaapad, Sora Bulq and Depa Billaba, fell to the Dark Side.
Although there is no reference to a Korun Sith, or a Korun dark side practitioner in general, they would be a strong candidate to produce one. Mace Windu describes it thus, from the Mathew Stover novel Shatterpoint:
I don't see Vastor as evil. Not as a truly bad man. Yes, he radiates darkness - but so do all the Korunnai. And the Balawi. His is the darkness of the jungle, not the darkness of the Sith. He does not live for power, to cause pain and dominate all he surveys. He simply lives. Fiercely. Naturally. Stripped of the restraints of civilization.
He is less a man than he is an avatar of the jungle itself. Dark power flows into him and out again but it does not seem to touch him. He is a savage purity that I might envy, were I not a Jedi and sworn to the light.
Black is the presence of every color.
He doesn't make the darkness, he only uses it. His inner darkness is a reflection of the darkness of his world; and it darkens the world around him in turn. Internal and external darkness create each other, just as do internal and external light: that is the underlying unity of the Force.
Windu is describing the nature of the Korunnai, his own darkness, and its connection with the Force in general. He saw his conflict with Kar Vastor as his own spiritual fight with his Dark Side, particularly as he had to face his own former Padawan, Depa Billaba, who had fallen to the Dark Side. The Korun therefore would be strong candidates to produce a black Sith, though they have not apparently done so in the available media.
N.B.1. There is another species, in the Star Wars Universe, with black people who are Force-users, and those are the Near-Human Tholothians; in Canon there are three Force-users from this species, Adi Gallia, Stass Allie, and Katooni - they were all Jedi, but they are another example of a species with black people who had Force-users amongst them.
N.B.2. Another black Force user, from an unidentified species, was Tu-Anh, but she again was a Jedi, not a Sith.
There are no black Sith Lords in canon.
In Legends, there are at least two human Sith Lords with dark skin like Mace Windu's. Both are from the video game Star Wars: The Old Republic, which takes place over 3,000 years before the events of the films.
They are from the Sith Empire, which existed before Darth Bane and the Rule of Two. There were many Sith Lords at that time and there are undoubtedly others which are black.
(It is worth noting that, in Legends, there is a Sith species. All members of this species have red skin.)
In Star Wars the Canon Universe, no the are not any Black Sith Lords. I don't know about Legends though.
All Canon Sith Lords:
If I am not color blind, Darth Vader is black from top to bottom.
Note: Darth Vader is a cyborg and his suit is integral part of his being and identity without which he can't survive.
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