Sterilize jars: Boil jars and lids for 10 minutes, then air dry.
Mash berries: Place blackberries in a saucepan and mash until smooth or chunky.
Add sugar & lemon: Stir in sugar and lemon juice until combined.
Cook jam: Bring to a boil over medium-high heat, stirring often. Boil 15–20 minutes until thickened. (Add pectin if using.)
Test set: Place a spoonful on a cold plate — if it wrinkles when pushed, it’s ready.
Skim foam: Remove any foam from the surface.
Fill jars: Ladle jam into jars, leaving ¼ inch headspace. Wipe rims and seal.
Seal & store: Process jars in boiling water bath for 10 minutes. Cool and store in a dark, cool place. Refrigerate after opening.
