Let's assume for a minute that gun control actually works to deter crime (from my research it does not, but we're pretending it does for this exercise). Let's also assume that reducing the number of African Americans murdered by gun crime is a major social priority (libertarians oppose all initiation of violence, so we're all fine with this). Finally, let's assume that any new gun control measures that happen right now can't go as far as a full-on gun ban but must be some kind of partial/conditional ban for political reasons (I don't think any reasonable person sees full population disarmament happening in America very soon).
It is an indisputable fact that the vast majority of African American gunshot victims are shot by other African Americans. Therefore, given this fact and our three assumptions above (which I think is fair to say that most liberals accept), there is only one "reasonable" gun control action available to the government:
Disarm the African-Americans.
This is, of course, a horrifically bad idea. It is also the most logical one if we accept the notion that gun control legislation saves lives.
For the record, I do not agree with the idea of State gun control. I believe all Americans regardless of race, gender, etc. who can be armed should be armed. It's harder to shoot people who can shoot back.