If anything, Black Marsh would be the only Province to not be featured in a game. It's so dangerous no Mer or Men have lived their in centuries. The last Men to live there were the Kothringi Tribesmen and they were driven out by a horrible flu. So, in summation, the only realistic way to feature Black Marsh would be a Traveler in a boat passing by the shore, or an Argonian which would be the only race able to survive.
I disagree, there is no point in avoiding such an exotic and interesting place just for the sake of "biological correctness". Maybe at some point the air became breathable, maybe other races found a remedy or their organisms simply became immune to poison in time from the exposure. Maybe fighting the hostile environment will be a distinct part in the story, who knows? I wouldn't give away so easily a gem like Black Marsh, which cries mistery and adventure, based on the little things that we already know from the books.