Blackboard: Restrict Sections

When you go into your topic/section through the left navigation menu, you can click the icon to Edit Section:

From this menu, you should see the options on the left to Restrict Access.

This will open a menu pictured below. This feature can be a good way to hide parts of your course until they have completed all the prior work for example(using Activity Completion).  

Many other options outside of activity completion exist here such as making groups of students and separating content this way, or using release codes to manage access, etc.