Stir until sugar is dissolved. Continue cooking over medium heat until mixture comes to a boil. Stir every few minutes to prevent scorching.
Boil until jam is set (220 degrees at sea level and -2 degrees for every 1000 feet above sea level.. 1000 feet 218°). Jam needs to reach temperature in order to thicken. Stir often to prevent scorching. Remove from heat. Ladle into clean, sterilized jars leaving a ¼ inch headspace.
Wipe down rims and add canning lids and rings. Place in a hot water bath and boil jars for 10 minutes (adjust time according to elevation - see notes below)
Notes
Be certain to add some unripened, or reddish berries as well as the ripe blackberries. The unripe berries have higher levels of pectin.
Cooking time will vary depending on your elevation and your stove.
I used pint-sized jars (2 cups.) You can also use ½ pint size jars. Processing time will remain the same.
Be certain to adjust water bath times according to your elevation.
Altitude Adjustments for Canning above 1000 feet in elevation:1,001-3,000 – increase 5 minutes3,001-6,000 – increase 10 minutes6,001-8,000 – increase 15 minutes8,001-10,000 – increase 20 minutesPlease read added notes in post (above recipe card) for methods on testing jam.This recipe has been updated to reflect additional information and now includes lemon juice. The lemon juice helps the jam set. I have had success with and without the lemon juice, but this can vary depending on your blackberries. I recommend adding the lemon juice to help ensure your jam sets up. 8/3/21