This is an argument from ignorance which I'm surprised you are using because it is a fairly common fallacy to justify many of the WoL revelations (the Overmind being always a slave thing etc) as well. Why are you assuming that people would ask questions about the black guy? If the guy was a nobody, I can easily describe his behaviour as being one of "combat stress reaction" (shell shock) and/or that he's an apathetic badass who realises the effed up situation he's in no matter what he does so why not chill out to some tune before going? Why? Because people are different and do not always behave the same way. There's no secret agenda behind it as you're making it out to be.
The black guys is as non-descript and no more special than any other Terran we see in all other cinematics featuring hapless Terrans. Jjust because he's black and that we see no other black guys in Starcraft shouldn't make one take a flying leap that any black man you see is also going be the only black main character we do see. There's a word for that sort of thing...
Also, he doesn't just "disappear". It is implied he dies just like the white guy as they're right next to each other when the Zerglings converge on their trench from both sides. If you're so inclined to think along those lines, we actually don't see the white guy get ripped apart either, so that must mean he's some sort of special character too, right? Just like whether the likelihood of some random black guy's identity actually being Duran, the answer is no in both cases.