Black Dwarfs were black-bearded Narnian Dwarfs.
Black Dwarves were, like all Narnian Dwarfs, strong, clever, and brave, but they also tended toward sullenness and vindictive, long-lasting anger.
The best recorded example of this temperament is Nikabrik, but there are examples of both good and evil traits existing in Black Dwarfs.
For more on the temperaments, skills and history of Narnian Dwarfs in general, see Dwarf.
In the 2005 film, The Lion, the Witch and the Wardrobe, Black Dwarfs were seen in the White Witch's camp where they help build up many swords for each creature on their side. They were present at Aslan's Sacrifice, and in the First Battle of Beruna where some of them use their bow and arrows to shoot down the Gryphons. (Three black dwarves, in a deleted scene, try to shoot down the phoenix.)
The White Witch's dwarf servant, who was named Ginarrbrik in the movie, was a Black Dwarf.In the 2008 film of Prince Caspian, Black Dwarves were among the Old Narnians who met at the Dancing Lawn, to discuss the arrival of Prince Caspian. Nikabrik was a Black Dwarf, who wanted to punish Caspian for the crimes of his people. Reepicheep reminded him, though, how his people fought alongside the White Witch, pointing out that you shouldn't blame the crimes of an entire people on one person.
When Caspian vowed to restore Narnia to the Narnians, the dwarves ended up joining forces with him and everyone, and then helped to gather weapons from the Telmarines. They later fought at the Telmarine Castle Night Raid, and the Second Battle of Beruna, where they fought the Telmarine army.