IDK, not necessarily. For one, why would Blackbeard be the only one who would be able to do this? Even something like the King's Haki, while rare, is possible for multiple people to have it. Second, the way Marco said (anime/manga, doesn't matter) it sounded unsurprising...like he knew something was up rather than it being something random like only Blackbeard's body is different from anyone else, especially when he said "average" people can't do it...but not everyone is just your average person in the OP world either.
I don't think this is something unique that only Blackbeard can do...although maybe his darkness DF power made it easier or something. Unless his DF is the only reason he was able to gain another DF in the first place, but I kinda doubt that.