I did that to focus on the murdering tendencies of Blacks and minimize the contribution of the mestizo group as much as possible. Graphing the murder rates per 100,000 population versus the percentage of Blacks gives a well-correlated run of points up the chart. X - 6.59434 where Y is the murder rate per 100,000 residents per year, and X is the percentage of the city population that is Black. An all-Black city would probably have an annual murder rate around 91 murders for each 100,000 residents. Obviously, no city can have a negative murder rate, so all that can be said of an all-White city is that it would probably have very few murders indeed.
X is once again the percentage of the city's population that is Black.
A typical all-White city would probably have about 590 serious violent crimes per 100,000 residents per year. A typical all-Black city would probably have around 4000 serious violent crimes per 100,000 residents per year.