Step 1: Prepare your canning jars and lids by washing them thoroughly. Sterilize jars by boiling them for 10 minutes in a large pot of water, keeping them hot until ready to fill. Warm lids in hot, non-boiling water.
Step 2: Gently wash blackberries, removing any stems or leaves. If using frozen, ensure they are fully thawed.
Step 3: In a large, heavy-bottomed pot, combine blackberries and lemon juice. Bring to a gentle boil over medium heat, mashing berries slightly to release juices. Cook for 5-10 minutes until soft.
Step 4: Add granulated sugar to the pot, stirring constantly until fully dissolved.
Step 5: Increase heat to medium-high and bring the mixture to a rolling boil that cannot be stirred down. Continue to boil, stirring frequently, for 10-20 minutes, or until the jam reaches its setting point.
Step 6: Test for set using the cold plate test: place a spoonful of jam on a chilled plate, return to freezer for 1 minute, then push with a finger. If it wrinkles, it's ready. Alternatively, use a candy thermometer to reach 220°F (104°C).
Step 7: Remove jam from heat and carefully skim off any foam from the surface.
Step 8: Ladle the hot blackberry jam into the warm, sterilized jars, leaving 1/4 inch headspace. Wipe rims clean.
Step 9: Place warm lids and screw bands on jars, tightening to fingertip-tight.
Step 10: Process jars in a boiling water bath for 10 minutes (adjusting for altitude if necessary).
Step 11: Carefully remove jars and place them on a towel-lined surface to cool undisturbed for 24 hours. Check seals; lids should be concave and not flex. Store properly sealed jars in a cool, dark place for up to 1 year. Unsealed jars should be refrigerated and consumed within 3 weeks.