Down By Riverside welcomes Cheryl Arena and her Wonderful Blues Band for a blues potluck at Arts Collaborative Medford. Arena is talented singer, songwriter and blues harp player -- she was one of only 30 artists featured in the book “Masters of the Harmonica.” She will be performing with her band featuring Eddie Scheer on drums, Ted Bukowski on bass, and Mike Dibari on guitar. They'll play great music to dance to, either in blues styles, swing dance styles, and…