Well, I finally did it! I joined Google+ and I tell you, I am confused by it. It's a bit overwhelming but I'm going to stick with it for awhile and see (I actually don't see where I have a choice...). But unfortunately I also discovered the following after joining:
Google + : No, but they will be prompted to create one when they attempt to comment.
Google+ : No. Although they can only comment using their Google+ profile or page, they can continue maintaining their separate Blogger profile. However, their comments will not link back to their Blogger profile, since the two are not connected.