First of all, sterilize your jars and lids in boiling water for approximately 5 minutes.
Then take a heavy-bottomed pot and combine blackberries and sugar. Let them sit for about 10 minutes to allow juices to release.
After that, add the lemon juice, butter, and a pinch of salt.
Bring the mixture to a boil over medium-high flame, and stir it constantly so that the sugar may dissolve.
Once boiling, reduce the heat slightly and continue to boil for 20-25 minutes, stirring frequently, until the jam has thickened.
To test the jam’s readiness, place a spoonful on a cold plate and put it in the freezer for a minute. If the jam wrinkles when you push it, it's done.
Remove from heat and carefully ladle the hot jam into the sterilized jars, leaving a 1/4 inch of headspace.
Seal the jars while they are hot, and allow them to cool to room temperature. Store in a cool, dark place.
