Blackboard has a tool called ROSTER. It allows the instructor to look up his/her students using a set of search options. Often, a course template, over which courses may be created, includes a tool link or button that opens this ROSTER tool.
Note that when you click on the ROSTER button or link, it opens a SEARCH FIELD, but does not show a list of your students. You must choose from the set of available options, then click the GO button, to find the student or students you want displayed.
If you want to see a complete list of your students or participants, you may want to instead go to the CONTROL PANEL, click on USERS AND GROUPS, then USERS.