AFAIK, the standard uniform in MD is the navy shirt (and navy jacket, especially for cool-weather games). If that is the case, then a navy plate coat (a plate coat is just a fancier version of a jacket, IMHO) shouldn't cause any problems. I'll also ask my association if a black plate coat would be approved for college games.
If I ever end up in the MLB, I'd like to at least try out the plate coat look occasionally behind the dish.