You're talking about very complex social issues here, there are no simple lines to draw. You looked at marriage but not at incarceration rates for example. You blame the Dems but the GOP played a part in the war on drugs, especially crack, that hit the Black community very hard.I wouldn't presume to know or judge. What do you base your assertion?
50 years ago, Black marriage and illegitimacy rates were approximately the same as for Whites. Look at how the Civil Rights marchers acted and dressed. Compare that with the hell holes that many of our inner cities have become since then. Diversity has become a code word for tolerating (if not elevating) dysfunctional and antisocial behavior in these Black communities.
The Democrats have a great political interest in maintaining these cesspools. It will take something from within (e.g., the Nation of Islam) to break this bondage and change the values and behaviors of these communities.