It's an indication of what liberal black CongressCritters think that's true, but otherwise it doesn't mean much
Historically southerners, both black and white, have been over-represented in the military. Inner city recruits, of whatever ethnicity, don't tend to stick around as long as suburban and rural ones. They also tend to be harder to train and create more discipline problems.
Here's my analysis of the problem. Unless you are already a patriotic person with an interest in the military, the prospect of Iraq & elsewhere doesn't look too great right now. Instead of black and white, let's look at in terms of Democrats and Republicans. Dems aren't as patriotic. I bet the rate of black enlistment tracks closely to Democrats in general. Once people get in, regardless of background, I think they become patriotic and like the services.