I was thinking perhaps redcurrant for the jam, but I'm liking the sound of a nice, tart rhubarb! Accepted 👍

Vipereon has most of us stumped, lol; I was thinking maybe pear and blueberry, ghostlyhamburger's husband suggested lime and blueberry, norakwami said maybe matcha again and cornflower petals (kinda grassy taste), and then ghostlyhamburger linked me to something called Honeyberries (which are kinda oblong, but otherwise pretty much the same blue as blueberries) as another blue food item. Subtle is definitely a good idea for Vipereon stuff, though, IMO; if there's something blue and-or green that's particularly associated with music, I might lean towards that.