The ability for Blackboard Collaborate customers to schedule their own large sessions (webinar mode) has recently been deployed and allows support for up to 500 attendees without needing to raise a support case. That is a fantastic development and removes a significant bottle neck.

https://help.blackboard.com/Collaborate/Ultra/Administrator/Release_Notes
https://help.blackboard.com/Collaborate/Ultra/Administrator/Manage_Sessions#webinar