In the context of America, black is an ethnicity as well as a race. Maybe we should have two different words to distinguish the two, but we don't, and that's the way the word is used. A racial-ethnic group is a group up people who share characteristics of lineage, culture, dialect, mores, norms, etc. Obviously, all talk of ethnic characteristics will be unfortunately general. Not every Italian likes espresso, not every American-urban-black person likes fried chicken, etc.
I think it is a very well supported statement to say that the "american-black-ethnic-group" that moved north into Detroit had levels of crime that the prior residents of Detroit found quite disturbing. And a major proportion of this crime was directed at white people in a racially charged manner. That is not to say that violence is an indelible attribute of all black ethnic groups, at all times, and all places. But high rates of violence was a property of this particular ethnic group, at this time, at this place. The movement of this ethnic group into the city, and the interactions of this ethnic group with other ethnic groups in the city, is important for understanding what happened. The Detroit story is partly a story of an ethnic-sectarian conflict, just as in Kosovo, Israel, post-bellum Atlanta, or innumerable other times and places in history.
What are the practical reasons is it important to recognize and talk about the ethnic nature of the conflict? I can think of a couple:
a) The current conventional wisdom (that I learned in college) was that white flight was due to an irrational and bigoted revulsion of black people. Thus the solution to preventing future white is to re-educate white people to make them not racist. The actual story, I think, is that white flight was a result of ethnic violence. White people feared black people moving into their neighborhood, because they had the quite rational and well-supported belief that this would dramatically increase the probability of black-on-white ethnic violence. The solution, then, is to take the same techniques of law enforcement and re-education that were used with tremendous success to reduce white-on-black ethnic violence and white-on-black racism, and use those techniques to reduce black-on-white racism and black-on-white violence.
b) If you are governing a city or a country that's experiencing a large influx of ethnic migration, and that ethnic group has cultural patterns that are causing problems, you need to work on strengthening your institutions to handle that influx. And if the influx is too much and too sudden, and your rate of building institutions too slow, you might want to think about ways to limit the influx, so you can properly assimilate the migrants, before the migration overwhelms your institutions entirely and you lose the city.