RINSE clean plastic containers and lids with boiling water. Dry thoroughly.
CRUSH blackberries thoroughly, one layer at a time. (Press half of the pulp through a sieve to remove some of the seeds, if desired.) Measure exactly 2 cups prepared fruit into large bowl. Stir in sugar. Let stand 10 min., stirring occasionally.
MIX pectin and lemon juice. Add to blackberry mixture; stir 3 min. or until sugar is dissolved and no longer grainy. (A few sugar crystals may remain.)
FILL all containers immediately to within 1/2 inch of tops. Wipe off top edges of containers; immediately cover with lids. Let stand at room temperature 24 hours. Jam is now ready to use. Store in refrigerator up to 3 weeks or freeze up to 1 year. Thaw in refrigerator.