It's not that negative - unfortunately, there are neighborhoods where people don't want to live in because of crime, gangs, homelessness, etc. Housing there is cheaper because the homes are not well maintained. Put up a new house and neighbors will not like it because if the property values go up then their property taxes go up - many are on a fixed income and can't afford it.
It also means they can't really fight the nearby gang activity because they have no other place to go.
Not saying you are wrong, but there is a lot more to it. That being said, there are options, and hopefully it leads to positive solutions for everyone.
There are some really bad black neighborhoods. They are portrayed that way on TV and movies, even glorified in many respects due to music, and therefore the perception is that black neighborhoods are bad.
However, there are bad neighborhoods from every ethnicity - depends on the city. If you look at Tulsa/Oklahoma City, Tucson/Phoenix, Miami, even LA's latino suburbs, the same can apply.
Cheap housing is not the primary focus for "good" people to move into a neighborhood. Gangs aren't necessary going to leave just because a nice, white family moved in next door. In fact, the gang is more likely to taunt them into either fearing them into staying in their house or getting them to leave. Gangs make money from fear and hate. They will hurt, even kill, to maintain their territory and reputation.
"It takes a village" is a cliche, but it works. Your multi-ethnic neighborhood worked because together they allowed the group as a whole to be positive. These neighborhoods are going to have to do the same, collectively. The local government can help, along with dedicated organizations like the CDA, to work with those neighborhoods to do that. But in the end, it's up to the neighborhoods. If they don't want to, not much is really going to change.
The alternative is a developer swooping in, strip all the properties outright, and just putting in what he wants with bribes to the city under the guise of "fixing the blight". It will no longer be a "neighborhood"; just a condo or apartment building, with tenants instead of owners.