The Black Numenoreans were the evil Numenoreans who worshipped Morgoth and who 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 Numenoreans waged war many times against the Gondorians and the Elves of the South. But, stop being a warmongerer. They are going to kick you out now.
In Numenor, the Black Numenoreans were the ones who dragged off innocent humans to Melkor's Temple and burned them to death. The Black Numenoreans were successful but then when Sauron got the Numenoreans to invade Valinor, the island of Numenor was flooded by tsunami, and the Black Numenoreans were eager to drag Elendil off to the Temple but he killed the Black Numenoreans who were after him, and ran to his ships.
The Black Numenoreans were the survivors who escaped over the sea with the Faithful when the tsunamis and storms hit Numenor and the island sank. The Black Numenoreans were thus the only survivors of the Numenoreans along with the Faithful.
The Black Numenoreans 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. Unlike the Easterlings, the Black Numenoreans were pure evil because they had no compassion for human life.
The Mouth of Sauron was a Black Numenorean and was one of their highest generals. The Black Numenoreans were the first minions of Sauron to return to Mordor in many occasions when the Dark Tower was rebuilt. The Black Numenoreans were empowered by Sauron and made stronger.
The Black Numenoreans were involved in the War of the Ring and the Mouth of Sauron was one general they had. But their powers were ended when the Ring went into the fire and the Black Numenoreans wavered and despaired and probably killed themselves or burned to death.