Blogs, discussion boards, journals, and wikis are all types of communication tools you can use in your course. Blogs are similar to discussion boards in the sense that each student has a space to reflect, write, and comment on one another's writing. This space is semi-public in the sense that other members of the course can see it, but it is not available to greater public.
Why use Blogs in my course?
You would want to use blogs when students have long written work, post a thought that is not so much a conversation as a topic to which others comment, or post information as a group.
For more information, see the Blackboard article about Blackboard Blogs.