Prepare boiling water canner. Heat jars in simmering water until ready to use, do not boil. Wash lids in warm soapy water and set aside with bands.

Assemble and measure ingredients.

Combine blueberries, honey, lemon juice, vanilla, and cardamom or cinnamon, if using, in a wide saucepan or jam pot. Bring to a boil over medium heat. Reduce heat and keep at a low boil, stirring frequently, continue cooking until jam is set and reaches near 220°F on a thermometer.

Ladle hot jam into a hot jar leaving ¼-inch headspace. Remove any air bubbles. Wipe jar rim. Center lid on jar and apply band, adjust to fingertip tight. Place jar on the rack in boiling-water canner with simmering water (180°F). Repeat until all jars are filled.

Water must cover jars by 1 inch. Adjust heat to medium-high, cover canner, and bring water to a rolling boil. Process half-pint jars 10 minutes, adjusting for altitude. Turn off heat and remove cover. Let jars cool 5 minutes. Remove jars from canner; do not retighten bands if loose. Cool 12-24 hours. Check lids for seal, they should not flex when center is pressed.