You can edit the privileges of a course or system role from the role's context menu. First, go to Admin Panel > Users > Course Role or Admin Panel > Users > System Role and for the role you want to edit (it doesn't matter if it's a standard role or one you created yourself, here you can see how to create custom roles), open its context menu and click Privileges.

You can then check or uncheck the privileges that interest you for the role. Be careful, there are many privileges. To simplify your work, there is a search box at the top. Type a word (for example, message) and the privileges containing it will be listed.

Here you can download a list of privileges and their definitions that Blackboard has published on its help site.