A) There will be no natives of Ravenloft this season.
Since at this point being a 'native' of Ravenloft is largely a role-playing rather than a mechanical benefit, it doesn't seem like this would be a big deal. The Far Traveler background from the Sword Coast Adventurer's Guide would work to justify such a background, though so, in my opinion, would many backgrounds from the Player's Handbook -- Darkon, for example, is much more similar to the Realms than it is different, and someone from that domain would likely fit into the Realms without too much difficulty (though if they were faithful to one of the Ravenloft faiths rather than a Realms faith, that might be both interesting and challenging).
B) This adventure is a remake of I6 Castle Ravenloft, and as such - predates the creation of other Domains of Dread (it is set in a time when Barovia is the only domain within Ravenloft).
This is a comment I would absolutely take with some pretty hefty doses of salt until we get guidance from the designers -- Perkins, Crawford, and Mearls have, when asked, given variations on "Ravenloft/Barovia is where it's always been", which suggests that Ravenloft as a setting still exists in 5th Edition D&D. It may not be exactly what you think it is** (for example, traveling to another domain is almost certainly outside the scope of both the Curse of Strahd hard-cover adventure as well as the Season 4 AL module series), but that doesn't mean the other domains don't actually exist.
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Pauper
** - For example, based on what's been previewed regarding the hard-cover adventure, it doesn't appear that the Barovia of 'Curse of Strahd' borders any other Ravenloft domains. While this may mean the other domains don't exist (yet), it may also mean that the 'Core' has been dissolved and every domain is now an Island of Terror, accessible only via the Mists. My guess, if I had to make one, would be the latter -- Darkon and Falkovnia and Bluetspur are all still out there, if you're playing in a home game and are foolish enough to go looking for them. It's doubtful you'll have the time in either CoS or the AL Season to do so, though.