this is my recipe which is very simple and doesn't require any pectin or sieving (as long as you don't mind a slightly soft set).
Take an equal weight of blackberries and sugar.
Juice of approx half - 1 lemon (depending on quantities - I shd think half a lemon would be ok for 600g)
Put into a pan together and simmer together on a medium heat, stirring continuously until the sugar has totally dissolved, then turn up heat to a rolling boil. Stir frequently.
Put a saucer into the deep freeze. When jam has been boiling for 15 mins or so, take a tsp of the liquor and put onto the cold saucer. Push the puddle with a teaspoon and if the surface wrinkles then the jam is ready. If you want a soft set then peer really closely for the merest teeniest little wrinkle. The more pronounced the wrinkle, the harder the set. If the jam is not ready then just return to the saucer to the freezer, continue boiling and try again.
If you want a really stiff set then you'll need pectin.
When done pour into a kilner jar.
If I am making for my own immediate use then I don't really bother to sterilise, I just wash the jar very well and rinse out with boiling water fresh from the kettle. You will need a half to 1 litre kilner jar for your quantity of berries. Half a litre should be ok, you might have a little left over that you could just leave to cool in a ramekin.
The jam keeps very well in the fridge and makes delicious apple pies, or stir a tsp into greek yoghurt for delicious blackberry yoghurt.