The phrase "black dwarf" refers to a theoretical celestial object that forms when a white dwarf star cools down completely. It no longer emits light or heat and appears black. Full definition
In theory, white dwarfs will eventually stop emitting light and heat and become black dwarfs. (space.com)
In the movie, Black Panther does momentarily spar with Thanos» oversized lieutenant Cull Obsidian (known as Black Dwarf on the comics page), just as he does in New Avengers # 9. (slashfilm.com)
Brown dwarfs were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too low in mass to sustain stable hydrogen fusion. (sciencedaily.com)