Both blogs and wikis offer a place for your students to build a body of knowledge. Since blog entries are ordered in time, blogs work best when you want the student to document the journey of building their knowledge. However, remember that Blackboard does not give you the ability to automatically gather all the blog entries about a topic in one place. So while it is easy to see the gathering of knowledge over time, it is not as easy to see the final result of that gathering. As wiki entries are organized as per page, they encourage the categorization and organization of knowledge. If seeing the result of the gathering of knowledge is more important than seeing the journey, then consider using a wiki.
One way to take advantage of both tools is to start by using a blog. Have the students blog about an area of study. Encourage (or require) them to read and comment on each other's entries. After students have built a significant body of knowledge in their blogs, make a coursewide wiki...