I have never tried to stream anything to ustream, but I doubt they will take jpeg/mjpeg streams. You probably have to push a stream to them, right? I don't think
Blue Iris is wired up to stream an auto cycle stream that way, but here is a kind of convoluted approach you could try.
Create a new camera and make it the generic MJPEG stream request type (near the very top of the camera type list).
IP address should point back at your Blue Iris server, like this: 127.0.0.1
For a path, enter
/mjpg/@groupname/video.mjpg where you replace
groupname with the name of your group.
If done correctly, you will now have a new camera defined in Blue Iris that automatically cycles between your other two cameras. I think you can probably configure that new camera to send itself to ustream.