When a student account is deleted from Bb, their grade information and uploads are dropped

From BbWiki

After students graduate, transfer, or leave a school they are removed from the Blackboard system. When a student account is deleted from Blackboard, any grade information or content uploaded by that user is removed from a course.

If an faculty member would like to keep all student content in a course it is important to archive the course and keep a copy of the archive file to maintain student grades and content.