Slow Cooker Blueberry Butter

The butter is outstanding. At least, the tiny bit that wouldn't fit into the canning jars for processing is outstanding. Does processing change flavors? I hope not.

I am a novice canner. This will be the first time I've stored my product on the shelf rather than in the refrigerator. I know sugar has preservative properties, but the berries were so sweet I just couldn't add any more.

Doubled everything about this recipe except the sugar. Cooked on high for 12 hours. Interpreted "1 lemon, zested" as "the zest from 1 lemon."