A better question is why you think that's a Democratic policy.
Because I can perceive reality.
You don't seem very good at describing the reality you supposedly perceive, though.
If any Democrat stated that it was his/her intent to keep blacks poor, uneducated, and dependent on government I missed it. When was this stated, and by whom? Seems to me that Democrats have done more than Republicans to promote opportunities for education for low-income and minority people, so there goes that one. They've also done more to promote job opportunities, so there goes that one. That leaves the dependent on government part. I guess when one party throws a lifeline while the other leaves people to drown, you could argue that the one throwing the lifeline is doing more to keep the person on a string, but frankly that argument, while valid, isn't terribly convincing.
Which is probably why blacks haven't been convinced by it.