I just question that number. I believe the percentage of the prison population back then that was black was higher than the percentage of the general population that was black. The latter number was more like 10%.
I'm not saying that the percentage of blacks in the prison population today isn't higher today than it was back then, and I'm not saying that the "Great Society" didn't have negative effects on the black family. It's just that there were probably more than 12-15% blacks in the prison population back then and way more than that percentage today.
Here are some numbers for you to look at, I didn’t go over them or the source’s accuracy but taking a quick glance it looked like you are right or might be.
http://theinjusticefile.blogspot.com/2011/08/prison-and-crime-records-by-race-black.html
If you look at them and form a summary or good conclusion ping me.