Alright first off the darksides are known as PUREBLOOD heartless emphasis on PUREBLOOD Xehanort created the emblem heartless which ironically have hearts from those lost to darkness, MX couldn't become a heartless though because people can only become an EMBLEM heartless whereas the purebloods come from the realm of darkness.
Nomura was right in the fact that heartless don't appear outside their homeland, they are not in any of the disney or original worlds, they are just behind the door of darkness...which as most of us hopefully all of us know that, that's where the heartless hang out.
Question Answered..I think.