Preparing the equipment: Before you start preparing your blackberry jelly, place canner rack in the bottom of a boiling water canner. Fill the canner half full with clean warm water for a canner load of pint jars. For other sizes and numbers of jars, you will need to adjust the amount of water so it will be 1 to 2 inches over the top of the filled jars. Wash jars, lids, and rings in hot soapy water; rinse with warm water.
Sanitize the jars, lids, and rings. Never plunge room temperature jars into rapid boiling water or they may crack. Place the jars in a large pot. Add 1-inch of water to the bottom, cover securely, and bring to a boil for 10 minutes. Keep the jars, lids, and rings in the hot water until they are ready to by used.
Preparing the blackberries: Gently wash, stem, and drain the blackberries (removing any stems, cores, dried-up berries, and leaves). Crush the blackberries with a potato masher, food mill, or use a food processor (If using a food processor, pulse to very fine chop) to lightly crush them. For best results, crush 1 cup at a time. Drip through jelly bag or cheesecloth-lined sieve to equal 3 1/2 cups blackberry juice.
Add dry pectin and sugar. Stirring to mix well. NOTE: Reducing sugar or using sugar substitutes will result in failure for the jelly setting up. To use less sugar, you must purchase and use the Pectin for Less Sugar or No Sugar. Stirring constantly, bring to a full rolling boil and boil, stirring constantly, 1 minute (a rolling boil cannot be stirred down). If you bring it back to a full boil fairly slowly (on medium heat rather than high) that will help reduce foaming. Remove from heat and quickly skim of any excessive foam/bubbles.
Ladle jelly into hot, sterilized containers, one at a time. Fill to within 1/8 inch of top if using self-sealing lids. Wipe rim of jar or glass with a clean damp cloth. Immediately place a hot lid and ring on top of the jar; tightly screw the ring on the jar.
Processing the jelly: Place jars on the elevated canner rack. Lower rack into the canner with the hot water. Add more boiling water, if needed, so the water level is at least 1 inch above the jar tops. Pour the water around the jars and not directly onto them. Cover the canner with a lid. Process in a boiling water canner for 10 minutes (start time when the water returns to a boil).
Altitude Adjustment for Boiling Water Bath Canner:
1,001 to 3,000 feet - Increase processing time 5 minutes
3,001 to 6,000 feet - Increase processing time 10 minutes
6,001 to 8,000 feet - Increase processing time 15 minutes
8,001 to 10,000 feet - Increase processing time to 20 minutes
After 10 minutes, remove jars with a jar lifter and place jars upright on a towel or cooling rack to cool completely. L eave at least one inch of space between the jars during cooling. Avoid placing the jars on a cold surface or in a cold draft. Let the jars sit undisturbed while they cool, from 12 to 24 hours. Do not tighten ring bands on the lids or push down on the center of the flat metal lid until the jar is completely cooled.
After jars cool, check seals by pressing middle of lid with your finger (if lid springs back, lid is not sealed and refrigeration is necessary). Put any unsealed jars in the refrigerator and use first.
Label jars and store the sealed jars in a a cool, dry, dark place up to 1 year. Refrigerate any open jars up to 3 weeks.
Yields 5 to 6 (6-ounce) jars.