The blue line represents the part of the satellite's trajectory after it traveled once around the earth, so it's basically the continuation of the 2nd stage track (except that the payload will have been deployed by then).
If you don't mind my paint skills, I've connected the two tracks in this image.
Because the stage/satellite is still being accelerated the track is at a higher altitude and because the earth rotates beneath it, it has shifted to the west from the earth's frame of reference.
Why does it suddenly go south and disappears?
It doesn't really go south, it just appears to do so, because the virtual camera is moving. As the track approaches the camera the faster it seems to be moving, until the camera flies right over it and it gets out of sight.
Shortly before it disappears you can even see the track being cut off. This is caused by the "near clipping plane" during the 3D rendering process, in which elements that are too close (in front of the viewing frustum) get discarded. In the animation more and more of the track exits the frustum until it's gone completely.