UPDATE: As of Aug 2017, this issue has been addressed. Users can now use a Collaborate Guest link weither or not they are logged into Blackboard Learn.
UPDATE: As of Aug 2016, this is still a known issue.
ORIGINAL POST: There is a known issue with Blackboard Collaborate Guest links cause some users to see a "Not Found" error page. This issue will be addressed by Blackboard Inc, and we hope to see it addressed soon. Until than, the work around is to log out of Blackboard Learn. For full details on the issue, please see the FAQ article "Collaborate Link Returns "Not Found" error page"