Inspired by this recipe at Saveur.com as published in Marisa McClellan’s cookbook Food in Jars. This turned out great although maybe a little thin. It was not sweet like jam or jelly and very full flavored.

Ingredients
8 cups puréed blueberries (about 3 dry quarts/1.7kg blueberries)
2 cups (400g) granulated sugar
Zest and juice of 1 lemon
2 tsp. ground cinnamon
1/2 tsp. freshly grated nutmeg

Instructions
Put the puréed blueberries in a 4-quart capacity slow cooker. Cover and turn it to low. After it has cooked for 1 hour, remove the lid and stir. Propped the lid open and told Tom to leave it alone as it suppose to be that way to allow the steam to escape.

This butter will need between 4 and 8 hours in the slow cooker. The time varies depending on how hot your slow cooker cooks. Check the butter at least once an hour to check the progress.
In the final hour, add the sugar, lemon zest and juice, and spices. If you want to speed the evaporation, remove the lid and turn the cooker to high. If you do this, make sure to check and stir the butter every 10 minutes to prevent scorching.

Prep’ed the lids and jars in a boiling water bath.

Once it was as thick as ketchup and spreadable purée the butter with the hand blender leaving some chunkiness.

Turn the slow cooker off and ladle the butter into the prepared jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath for 10 minutes.