If you're using the Chat or Virtual Classroom options in the Collaboration section of Blackboard, we recommend version 5.0 of the Java plugin for your browser for both students and instructors. Newer versions (such as version 6.x) have been known to cause problems with collaboration sessions.
You can download the version 5.0 of the Java plugin for Windows here:
http://www.ats.neu.edu/blackboard/java/jre-1_5_0_12-windows-i586-p.exe
(for Macintosh, please use the built-in java capability of your browser)