H/T Sunshine Mary
Now take a look; all ethnic groups show a decline starting not in 1964, when LBJ started the War on the Poor, but rather around 1950, when he was a junior Senator. Particularly interesting is the plunge in black marriage rates at this time. It doesn't correspond to the Great Society or even the Civil Rights movement--those were still in the future--but it does correspond well to both the second Great Migration and the graduation of the first soldiers studying under the G.I. Bill--soldiers who had served with all races in Italy and elsewhere with the 92nd Infantry and other units, and who had been welcomed as liberators there.
Of course, getting away from "Jim Crow" is a good thing, even if northerners were not altogether hospitable at all times(to put it mildly), but one thing that is likely here is that as blacks became a distinct minority in their new homes, they didn't have the clear support of institutions like family and church (those they'd left back in the South), and they also had to wait to establish themselves in homes before marrying and starting families. And, of course, being lonely and far from home, I'm guessing some did what lonely people of all races do....with or without a marriage license, and their families and churches weren't there to give them a nudge towards marriage.
And so perhaps, just as the great migrations of whites to the West resulted in a culture (excepting the Mormons of course) with less regard for traditional mores, marriage, and the like, perhaps the migrations of blacks to the north and west did the same thing....and the question, then, is how to fix the problem after decades of hurt.
A parallel note; the rise in crime rates in the inner city started soon after this....again, exactly what you'd expect from lonely people of any race, and it is worth noting as well that the race riots of the sixties all occurred in the north and west. Now granted, fear of "Bull Connor and company" may have suppressed things in the South, but Richard Daley was no slouch during the Democratic Convention of 1968, either.
The moral of the story, I think, is to welcome your new neighbors. It'll do them, and you, a lot of good, and it's in the Bible, too.