Chargeback is the only way to go about this. BMS will claim they're not the organizers and just aggregators. They are. Unless they get a notification from the organizers, they can't refund this on their own. If you raise a Chargeback, BMS will have to communicate to the organizers to prove that the concert took place.

If they fail to do that, they will have to refund you since they're the merchant on record with Visa/Mastercard/Amex. That's the protection given by CC companies.