Rinse and drain blossoms; place in a large heat-resistant glass bowl. Pour boiling water over the blossoms and let stand for 2 hours, stirring occasionally.
Strain and reserve violet liquid, pressing with a spatula to extract all possible color. Discard blossoms.
Measure violet liquid; add enough water to measure 3-1/2 cups (liquid will be blue-green). Stir in pectin, lemon juice, and sugar (the liquid will turn a violet color).
Pour into a large stainless steel saucepan; bring to a rolling boil, stirring constantly. Boil 1 minute.
Remove from the heat; skim off foam. Carefully ladle the hot liquid into hot sterilized half-pint jars, leaving 1/4-in. headspace. Remove air bubbles; wipe rims and adjust lids. Process for 5 minutes in a boiling-water canner.