The sole purpose of the black Confederates fad is to sell that the Confederate cause wasn't about slavery ("cause look, even blacks fought for the Confederacy!") and that the secession South was the rightful heir of the original Constitution and the idea of Life, Liberty and the Pursuit of happiness (because slavery was a better condition than primitive tribal Africa or the industrial serfdom in Northern cities).
Why is the creation of such new fads necessary? -- because by the 1980s or so the precepts of the original Lost Cause became such an embarrassment in modern society. Confederate apologists had to scramble for a new narrative that would resurrect some semblance or repectability in modern society -- meaning their personal reputations, as if they were themselves Confederate.
And btw no one alive today is Confederate. The real ones are all dead, and no one today is willing to make the sacrifice it took to be a real Confederate -- to abandon the benefits and comforts of U.S. citizenship. So there are only play-Confederates, and as to why they haven't done enough in their own lives today to be proud of is the fair question.
Besides which play-Confederates do not speak for real Confederates. Those who properly memorialize the sacrifice and duty of the real Confederates are the ones to listen to, not the ones who make up stupid s--- about what the Confederacy was.