Fair enough. I'll tell you what I think.
First off, I think your premise that either something happened to cause this or blacks are just defective is a false dilemma. I believe that the current disadvantages faced by blacks in the US are absolutely not in any way a result of blacks being genetically or racially defective.
And while I do think some of those disadvantages are caused by the racism of other groups, particularly white Americans, I do not think this is the biggest reason any longer. In my opinion, the black community int he US, which is an admittedly poorly defined term, is suffering a severe cultural crisis, which can never be solved until it is admitted that many of the obstacles to success for young blacks, young black males especially from what I see, are cultural habits long established in the black community.
The undervaluing of hard work in the educational process, the glorification of criminal violence and gangs, the shocking lack of responsibility shown toward women and children by black fathers are all enough to ensure that black educational and financial success would be less than in groups like Asian-Americans where we see the opposite habits. (When I look at white Americans, I'd say a large minority adopt the low class values and are generally referred to by other whites as white trash, while most have what we broadly term middle class values.)
Many of the worst cultural issues afflicting the black community have substantially worsened since the end of Jim Crow. So while I think it is unfair to judge the black community without accounting for the structural racism that oppressed them for the bulk of their history in the US, I think it is also naive to fail to hold them accountable for their own problems. It seems many people feel that all black failure is easily explained by the attitudes of white people, while others point only to the cultural failings of the black community. I'm willing to look at both as two sides of one serious issue.