When creating an Assignment in Blackboard, you have the option to assign it to each student individually or assign it to Groups. One member from each Group can then submit the assignment for the whole Group.
Once the Assignment is set up as a Group Submission, it will only be visible to the selected Groups. If a student has not been added to one of the assigned Groups they will not be able to see or access the assignment.
The instructor will also not be able to see the Assignment when using Student Preview mode if their preview user account has not been added to one of the assigned Groups.