This strikes me as a bit like blaming universities for taking too many pupils from independent schools. It's not the fault of the universities, or the armed forces, that not enough highly educated and aspirational black young people either exist, or apply to them. It goes much further back, into the education system.
Of course, if there were concrete evidence that black personnel were discriminated against for promotion that would be another thing. But I very much doubt it, not in the last 20 years at least. Before that, perhaps.