It was “Blue Oyster Cult”. The song they were using was “Don’t Fear the Reaper” - probably the one song you’d NEVER pick to add cowbell to!!! :)
BOC was my very first concert (grew up in small town Montana, not much of a concert selection there). lol.
I loved how they started the song on SNL and then Will just came in wildly, unabashed. Excellent. Wish I could see it again. It has been a wild ride and it is going to get even more bizarre . . . thank God for the cowbell! :-)