Place blueberries in large bowl. Crush with potato masher to split open berries. Pulse with hand-blender (also called immersion-blender) — or transfer to food processor — to further break up blueberry skins into smaller pieces to preferred texture.
Into blueberry mixture in bowl, stir in sugar, zest and half the lemon juice. Taste. If more tartness is preferred, add remaining lemon juice. Whisk in cinnamon, if using. Let stand 15 min.
Add pectin. Stir 3 min., to blend in pectin and ensure sugar is completely dissolved, no longer grainy.
Divide blueberry mixture into four 1-cup (250-mL) freezer-safe containers with tight-fitting lids, leaving 1/2-in. (1-cm) head room for expansion during freezing. Cover loosely with lids and let rest at room temperature 12 hr. to set. The jam is ready to eat after resting. Seal lids tightly and store refrigerated up to 3 weeks. Or freeze up to 6 months.
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