A course ID in Blackboard is typically a 13-or-more character alpha numeric code comprised of the letter 'B', the CRN number, a dash, and the term. Non CRN courses have other ID types such as P_name or T_name.
e.g.: B20001-200520 or P_FacultyForum or T_JoeTestSite
To locate the ID of a particular course, go to the top of the course in the student view of Blackboard where the course name : course id is displayed. The course ID is circled in the screenshot below:


Alternatively look in the the control panel of the course and look for the ID in the blue bar at the top of the page.