First login to Blackboard, check out the Blackboard Orientation Video, and then View Your Course in Blackboard to be able to Navigate within your Course.
How to Post to the Discussion Board
Once you’re in your course click on the header or button labeled “Discussion 
Board”.
If there is no button click “communication” link under “Tools” and then click “Discussion Board”.
Click the Link for the discussion board that you need to post to.
At the top of the page, Click the "+ Thread" button.
Type in your subject title, the body of your post and click "Submit".
Congratulations! You’ve just made a discussion post!
Saving a Post
You can save a Post in Blackboard by clicking "Save" instead of "Submit".
Resuming a Saved Post
Your saved post will have "(draft)" next to the title. In order to resume your draft, click the link to the post, then click "Modify". This will place you back into edit mode for the Discussion and then you can submit the post when you have finished your edits.
Discussion Board Errors
If the "+ Thread" or "Reply" buttons seem to be missing in the Discussion Board, double check with your instructor to ensure that the Discussion Board is set to be interactive for students. If you are receiving errors while attempting to post in Blackboard, please view the Java Script Error webpage to ensure that you have the latest Java Script installed on your computer.