Blackboard Collaborate does not currently have the automatic closed captioning feature, however, there is the opportunity to make an attendee a captioner. Captioners type what is being said during a session. Other attendees can view what is being typed in real time. You can have multiple captioners for multiple languages. More on live closed captioning.
Google Chrome can also provide users with automatic captions of videos that they watch, including Blackboard Collaborate sessions, however, they do not caption you as the user (your own speech is not detected, only incoming audio speech). Google provide instructions on how to turn automatic captions on within their Chrome Help site.