…is cinnamon. A scant 1/2 teaspoon per quart of blueberries (half that amount if it’s a very fresh cassia cinnamon) gives blueberry jam amazing, subtle complexity. It’s a flavor that’s hard for people (even a lot of experienced cooks) to define exactly, which makes it a great trick for secret ingredient recipe warriors (What, that? Oh that’s just a little recipe momma used to make!). Works equally well with blueberry pie or cobbler.