This article explains the differences between blogs, wikis, journals, and discussion boards in Blackboard 9.1.

Discussion Boards: Discussion Boards are the most common of these four tools. Discussion Boards allow users to post and reply to messages. Replies that are associated with the same post are grouped together, creating message threads that can be expanded and collapsed. Generally, the course instructor controls the topics. Users can subscribe to forums or threads and receive email when there is new activity.
Potential Uses: Popular tool for online discussions. Consider class debates, team discussions, role plays, etc.

Blogs: Blogs allow participants to post a chronological series of entries on a particular topic, either individually or shared. Newest entries come first in the list and users can add comments to blog entries. Group blogs can be viewed and edited by all group members. Blogs are less structured than the Discussion Board. The format is more open and conversational in style.
Potential Uses: “What we did/will do in class” saves the instructor of having to answer individual inquiries, online discussions about related topics, “muddiest points” about what was covered in class.

Journals: Journals provide a place for students to write. Journals can be kept private between the instructor and the student or shared with the class. Only the instructor and author of the journal can add comments. Group journals can be viewed and edited by all group members.
Potential Uses: Reflection personal growth throughout the semester, record lab results, document clinical experiences, communicate “muddiest points” that are private.

Wikis: Wikis are a collaborative space where all students can view, contribute and edit content. Wikis can be viewed, edited, and commented upon by all users in the course. Can be viewed and edited by all group members Each wiki contains a History detailing all the changes made to the pages.
Potential Uses: Course glossaries, creative writing, group research projects, and student created study guides.