Are you using a Mac running Mountain Lion v10.8.4? If so, then Blackboard has released a special app called the "Collaborate Launcher for Mac" which is now required for Mountain Lion v10.8.4 and later.
From Blackboard:
For Mac users who are on Mountain Lion (v10.8.4 or greater), Blackboard has made a major change to the way Collaborate sessions are launched. Mountain Lion users must now download and use a special app called the "Collaborate Launcher for Mac." Basically, it includes it's own custom version of Java, which means that Mountain Lion users no longer need to install Oracle's version of Java in order to run
Collaborate sessions.
Included below are links to support articles about the new Collaborate Launcher for Mac. We strongly encourage all Mountain Lion users to review this information so as to avoid any unexpected problems the next time you need to join a Collaborate session.