Duregar
The Duregar initially were not well regarded in Dwarven societies and even beyond, seeing them as foes throughout history (a relationship akin what the Drow have). The first Duregar were initially criminal outcasts sent down into the Underdark to live out their lives, where overtime they grew to form broader societies and even states.
During a period of time they were enslaved by the Illithids or “Mind Flayers” alongside the Drow. The Duregars following this freed themselves from their Illithid masters in a mutual alliance with the Drow to release themselves from their own evil masters’ grasps. Following this the two peoples now consider each other great allies within the Underdark.
While some might venture onto the surface to learn of societies above or even barter alliances, the vast majority reside in the Underdark, living in both Drow/Duregar shared cities or even their own settlements.
Today a Duregar trying to take part in Dwarven society might not meet the same animosity as they once did before, but with many years passed developing cultures differently, many are happy living in their own society rather than trying to rejoin the Dwarves, seeing themselves as a distinct people from them. Thus the misnomer of “Dark Dwarf” or “Grey Dwarf” might be found offensive to some.