This same method and process can be used to make jam from other berries too, though ones lower in pectin may need some pectin added. How to make homemade pectin is HERE, or simply combine high and lower pectin fruit in a jam, by adding some apple or quince to berries when cooking. Removing berry seeds is optional.
Note: expect this to be a somewhat messy process - wear an apron, anticipate some juice being spilled or splattered, and keep your favourite white dress far, far away! :-)
Place blackberries into a large pot. Gently heat until boiling, and mash with a potato masher to release juice and help separate seeds and pulp. Simmer on low for 10-15 mins, mashing and/or stirring from time to time.
Place a large sieve over a bowl, and pour the berries into it. Use a wooden spoon or spatula to force the berry pulp and juice through the seive, leaving behind the seeds. Work on it until you've got as much pulp as possible through.
Measure the resulting blackberry pulp/juice mix and place in a good sized pot. For every 600ml of berries, add:
1 TBSP lemon juice.
Heat and stir until sugar is dissolved, then simmer and stir regularly until set point is reached. This is when a little jam dropped onto a cold plate keeps it's shape when cool and feels quite jam-like when you run a finger through it. It will stiffen further once bottled and cooled though, so don't expect it to be totally set in the test. Sometimes I think the jam left on the spoon I dipped into the pot to test with is the better test - when the remaining jam is sticking to the spoon and not running off or moving, it's good to go. :-)
Ladle jam into hot, sterile jars, and seal. For more on how to prepare jars etc, refer HERE.
(I started with 850g of blackberries, ended up with 600ml of juice mix, and resulted in two 400g jam jars full)