Place blueberries in the pot of your slow cooker. Cover with the lid and turn slow cooker on HIGH. Cook 1 hour*. Remove lid and stir. (If starting with blueberry puree, skip this step)
Cover the slow cooker and cook on LOW for another hour. Use an immersion blender to puree the blueberries. (If you do not have an immersion blender you can either transfer the blueberries to a blender or food processor and puree. If you started with pureed blueberries skip pureeing)
Keeping your slow cooker set to LOW, cook for 3-4 hours, giving the blueberry puree a good stir every hour or so, and making sure you replace the lid keeping it cracked. After 3 hours check your butter. If it is about at the consistency you want, stir in the sugar, honey, lemon juice, lemon zest, cinnamon, nutmeg, and salt.
Again replace the lid, keeping it cracked, and continue to cook on LOW for 1 final hour. If the butter is not as thick as you would like after the final hour, set the slow cooker to HIGH, remove the lid completely, and cook for 30-45 minutes, stirring every so often to prevent the butter from burning on the bottom.
OPTIONAL: If you want a silky smooth butter: Use a food processor to puree the butter. Place a fine mesh strainer over a large mixing bowl and use a ladle to transfer butter to strainer to remove any large particles.
Transfer butter to Jars and secure with a lid. Place in the refrigerator. Butter will keep in the fridge for one week.
Yield: 3 Pints