They claimed to be the true Horde. They may yet prove it.
The Blackrock orcs seem, on the surface, to be a relic of a bygone age. A tribe of orcs holding on to a past swept aside by Turalyon's hand clutching Lothar's broken sword, a defeated remnant of Gul'dan's legacy. Once led by Blackhand the Destroyer, the Blackrock clan rose to prominence when Blackhand became Gul'dan's proxy as Warchief of the newborn Horde. And it remained central when Orgrim Doomhammer, Blackhand's second in command, slew Blackhand and seized power, for Doomhammer too was a member of the Blackrock clan. After the final defeat of the Horde atop Blackrock Spire, it seemed certain that the Blackrocks would trouble Azeroth no more.
Yet Blackhand's sons Rend and Maim, who had served Doomhammer even after he killed their father as leaders of the splinter clan the Black Tooth Grin, led the Blackrocks into the mountain that bore the same name and set about rebuilding them. When Teron Gorefiend came to Blackrock Spire, the brothers Blackhand refused his call to join Ner'zhul's Horde, seeing themselves as the true inheritors of Blackhand's legacy. In time, the Black Dragonflight came to the mountain, attracted by the reds still held in bondage there, and Nefarian brought the Blackrocks into his service. Maim Blackhand died in the war with the Dark Iron Dwarves to determine which force would rule the mountain, while Rend died when Thrall sent members of the Horde to kill him for his claim to the title of Warchief that Doomhammer had bestowed upon the shaman.
And yet, the Blackrocks were not done. Ironically, it may have been one of Thrall's most loyal supporters who brought one of Garrosh Hellscream's most dangerous enforcers into the fold.