If you want to have your subject in focus and blur the background as much as possible, zoom in to your maximum tele focal range and move closer to your subject.
Longer the focal length, nearer the subject to you, further the background to the subject, larger the aperture => better background blur effect.
You may not get the creamy blur that can be created by a good prime lens but I guess you won't be that demanding at this point?
If you have the time to borrow/rent a 2.8 lens or a flash within tomorrow morning to afternoon
since the concert start @ 6.30pm, that will give you more advantages .. Cheers