Well, I actually think it makes sense to specify a negro here. We're talking 1960s television and that may have been the only way the writer (or whomever) could guarantee ethnic diversity. Leave the specification out and the casting department may think "Oh, a Commodore must be white. Kirk can't have a black superior officer."
Of course, that may not be the reason at all. It's a nice theory.
A casting note, undoubtedly. I'm just chuckling at the phrasing, which sounds blatant and awkward to 21st century sensibilities.