Detroit certainly wasn't nice. But I don't think it was really impacted that much by the crack boom or even "gangsta rap" or gheri curls. It was bad in the 70's, 80's, and 90's. And now too.
Personally, I think the real reason crime has decreased since then is only because so many people have left the city. In the 70's, there was over 1.5 million people in the city, in the early 90's, there was about 1 million people. Now there's probably 800,000 people in the city (that's probably a generous estimate), so yeah there's less crime. But is it any safer, or is it just cuz there's less people out there to commit crimes?
Also, considering the city has one of the highest unemployment rates in the country, one of the lowest high school graduation rates (at least as far as DPS, I don't know much about the new charter schools to be honest) and one of the highest percentages of population under the poverty line, and that makes people desperate. And desperate people do desperate things.