For a collaborative blog, I'd suggest either a Blogger or Wordpress blog. Though Blogger is getting tangled up with Google+, for students with Gmail accounts already, the single log-in is advantageous. You could be the blog owner, and students could be contributors (co-authors), or, to ease them in, they could have only commenting privileges that you could moderate. However, screening comments in advance can impede interactions and responses among contributors, especially if you're not monitoring the blog 24/7.
I believe students would have to have Diigo installed on their computers to facilitate cooperative bookmarking of Internet sites. You'd probably want to set up (a) Diigo group(s) for them, with recommended tags. For example, I own a Weblogging in Kumamoto (WinK) group, and have an RSS feed for bookmarks tagged "wink_students" in the sidebar on a writing course blog. However, students don't do the bookmarking for that; other teachers and I do. You and other group members can share annotations and highlights publicly or privately (with particular groups).
With a Diigo group, and student invitees, discussion forums also will be available. So course blogs might be unnecessary, if you go the Diigo route. Carefully threaded discussion posts may be better than flat comment threads on target blog posts. (I'm not sure yet, but Blogger may have begun threading replies to particular comments.)
I hope this responds satisfactorily to most if not all of your recent inquiries. If not, please let me know in (a) comment(s) on this post.