You should probably work with Bluejeans support on this one. They can see what is happening when your call initiates and see if the call negotiation/setup process is completing properly. Often though, when people experience black screens or no video/audio, you're perhaps running into an issue with a firewall.
I have this issue a lot. The only way I can find to resolve it is to power down the Polycom GS then reconnect to the BlueJeans meeting. We have this issue a lot and no one seems to have found a resolution yet. Just sporadically works.