About the black things, they're most likely portals of some sort because you come through one to get to that place.
I don't realy know about if Xehanort created the Heartless there. It's possible, and, now that I think of it, is probably correct. He said he had a machine, and that's the only random machine thing we don't know about in the game.
Also, remember what it said about Kingdom Hearts and the 2 Keys and whatever? I know it's nothing shocking or new, but I still find it interesting to read.
Oh, and did you notice when playing through KH1, Leon, Yuffie, and the others kept talking about how their world, (Hollow Bastion) was destroyed by the darkness, being the reason why they're in Traverse Town.
Well, I have a pretty strong feeling that the events that occured in Birth By Sleep are what eventually caused Hollow Bastion to lose it's heart.
What do you think?