Rinse the lemons in hot water, peel thinly with a sharp knife and cut the peel into thin strips. Squeeze the juice.
Rinse the blueberries.
Mix the blueberries, jam sugar, lemon juice and lemon peel. Bring to a boil over high heat for about 4 minutes. Remove from the heat and stir in the liqueur. Pour the jam into hot rinsed jars, seal tightly and flip upside down for about 15 minutes. Flip right side up and let cool.
Serve over yogurt, if desired.