While I would not be surprisd if blacks served as stewards, I would not be shocked if they served in other capacities, especially in times of emergency.
I still believe that in submarines it would be not possible for any sailor to not fill both his described section, and other roles as well. Most submariners of the era were qualified in more than one "job". The crew is too small, and being voluntary each must simply must be able to "multi-task" (in today's lingo).
I just received an email regarding this thread, so very satisfying as it is one of the very oldest threads on the entire site. My original post was a shocking 18 and a half years ago! The email contained some info regarding a Mr. Robert Wise, who was a black and served on US submarine(s) in WWII. I'll see about gathering more info, and hopefully having the emailer join the site to participate.