^ I don't think Starfleet would do that. You can't have every officer trained in every line of work. It just doesn't work that way. There must always be specialization. You can't pick starship crew and assign them to ground combat willy-nilly - it'd be like sending a Walmart security guard to fight in Afghanistan.
The guys in the black uniforms could be Starfleet's equivalent of Navy SEALs - naval personnel, but trained in missions like this. I do like the concept of a Starfleet Marine Corps though. I'd prefer to have dedicated
ground combat divisions.