Another characteristic of Blacks, as a group, is there amazingly strong proclivity for voting Democratic. 93% voted for Obama, for instance. No other group comes even close to that level of group-think and block voting.
Once a city gets to being 30% Black or so, it's going to be Democrat-ruled on account of this. Once it gets to 50% it's going to Black Democrats running things. In Detroit that meant Mayor Coleman Young, a corrupt, vicious, thuggish anti-White racist who ruled Detroit for 20 years. John Conyer's wife, Detroit City Counselor, in prison for bribery. Kwamie Kirkpatrick, another former Mayor of Detroit -in prison for Felony corruption. William Hart, the Chief of Police - stole $2.6 million dollars from the evidence locker.
Portland, Oregon (6.3% black) has had a string of corrupt Dems too, none Black either. But it's a different type of corruption. Goldschmidt (who went on to be Governor) was banging a 14 year old babysitter when he was mayor. Disgusting, but not really destroying the city in the process. The papers covered up for him till the statue of limitations ran out. Vera Katz, his successor, wasn't ever accused of financial corruption. The last guy, Sam Adams, gay fellow, was banging a 17 year old boy shortly before his election to Mayor, but no one would testify against him so he claimed they "just kissed", and got away with it. Again, gross, but not the same as looting the treasury.
Once found out corrupt R's always have to go. But even most corrupt D's have to go, unless they are black. Look at Rangel, Conyers and Marion Barry, for instance. It's just not relevant to Black voters, apparently.
Years of this helped destroy Detroit, the willingness of the citizens to elect super-corrupt Black politicians.
There are some Southern cities that are greater than 30% that have elected Republicans. Chattanooga, TN is about 35% black and has recently had two Republican mayors, though the current one is a Dem. Montgomery, AL, which is 54% black, currently has a Republican mayor. I’m sure there are some examples of smaller cities with similar demographics where this also true. This may be because the white population in these areas, like much of the South, is almost entirely Republican. For example, the white vote in Mississippi and Alabama went 90% and 85% respectively Republican in the 2012 Presidential Election.