The Black Númenóreans are major antagonists in The Silmarillion and supporting antagonists in The Lord of the Rings.
They were the evil Númenóreans who worshipped Morgoth and set themselves up as dark magic practitioners and warlocks in the Second Age of the Sun. Feared by the Elves and the Faithful, the Black Númenóreans waged war many times against the Gondorians and the Elves of the South.
In Númenór, the Black Númenóreans were the ones who dragged off innocent humans to Melkor's Temple and burned them to death. The Black Númenóreans were successful but then when Sauron got the Númenóreans to invade Valinor, the island of Numenor was flooded by tsunami, and the Black Númenóreans were eager to drag Elendil off to the Temple but he killed the Black Númenóreans who were after him, and ran to his ships.
The Black Númenóreans were the survivors who escaped over the sea with the Faithful when the tsunamis and storms hit Númenór and the island sank. The Black Númenóreans were thus the only survivors of the Númenóreans along with the Faithful.
The Mouth of Sauron was a Black Númenórean and was one of their highest generals. The Black Númenóreans were the first minions of Sauron to return to Mordor in many occasions when the Dark Tower was rebuilt. The Black Númenóreans were empowered by Sauron and made stronger.
The Black Númenóreans were involved in the War of the Ring and the Mouth of Sauron was one general they had. However, their powers were ended when the One Ring went into the fire, and the Black Númenóreans wavered and despaired and probably killed themselves or burned to death.
The Black Númenóreans were of many factions and organizations, they all worshipped and performed dark magic, performing human sacrifice in Middle-earth, and Sauron was always their master, even though they worshipped Sauron's own master, Morgoth. Unlike the Easterlings, the Black Numenoreans were utterly callous and heartless because they had no compassion for human life.