Wash and drain the blackberries. Run them through a food mill to remove the seeds. Reserve the blackberry puree (you should have about 5 cups) and discard the seeds (I give them to our chickens).
Place the blackberry puree in a large pot and add the sugar and lemon juice.Bring the mixture to a boil, stirring regularly, until the jam reaches 220 degrees F. I use an instant read thermometer.
Ladle the jam into sterilized jars leaving 1/4 inch headspace and wipe the rims clean before sealing.If you're not sealing the jars the jam will keep in the fridge for several months.If you're using the water bath canning method process the jars for 5 minutes. Carefully remove the jars, let them sit undisturbed for 24 hours, then store in a dark cool place. Jam will keep for up to a year.