I agree with you Jim Crow ended, and U.S. employers and institutions are no longer allowed to be racist, under the law. (Individuals still can be racist since we do not literally police their thoughts, nor should we.) However, I think you are overlooking some other issues.
Black families are poorer than white families overall: much poorer. Being poorer consigns them to cheaper homes, in poorer neighborhoods with a smaller tax base and worse schools. Worse schools translates into fewer educational opportunities, in a society where education levels and credentials are paramount. That means they can't compete for the best jobs. If they can't compete for the jobs, they stay trapped in poverty. And the cycle continues.
I'm not saying any of this is *actively* racist, but these are echoes of the state-backed racist policies that ended decades ago. Blacks were not much allowed to build up family wealth under Jim Crow, and certainly not under slavery.
The bigger issue is that our 21st-century economy just doesn't work for people anymore and the ladder of social mobility has kind of fallen off.
I would consider an example of "Advancing a political cause" in this context to mean advocating for increased funding for public schools, or giving families in poor inner-city neighborhoods (regardless of race) vouchers to move their kids out of poorer school districts. I believe you can design policies in race-neutral ways that will still benefit both black folks and poor white folks, and that is fine. You're still helping black people at that point.
I don't think that supporting #BLEXIT qualifies as actually helping them. I've seen no evidence that blacks are abandoning the Democratic Party in droves. Sure, you can point to a black Republican here and there but the Democratic Party has many more black people in every level of leadership.